WO2018149866A1

WO2018149866A1 - Fabric preservation of uncemented sand samples

Info

Publication number: WO2018149866A1
Application number: PCT/EP2018/053657
Authority: WO
Inventors: Miriam MARTÍN RUÍZ; Elena Margarita ESCOBAR ÁLVAREZ; José ALVARELLOS IGLESIAS
Original assignee: Repsol SA
Current assignee: Repsol SA
Priority date: 2017-02-15
Filing date: 2018-02-14
Publication date: 2018-08-23
Anticipated expiration: 2019-08-15

Abstract

The present disclosure relates to methods for preserving the inner structure and the mechanical integrity of uncemented sand samples, and to preserved uncemented sand samples whose inner structure has been preserved by the herein described process.

Description

Fabric preservation of uncemented sand samples

This application claims the benefit of European Patent Application

EP17382072 filed on February 15, 2017.

The present disclosure relates to methods for preserving the inner structure and the mechanical integrity of uncemented sand samples.

BACKGROUND ART

In the oil & gas industry, core analysis is known as the laboratory

measurement of the physico-chemical properties of rock samples of recovered core for multiple purposes. Core samples are considered as one of the most important sources of information on subsurface rocks. Laboratory core analysis provide important information for technical and economical evaluation of oil recovery potential. Petrochemical data from core depends strongly on the size and shape of intergranular porosity, and for core data to be representative of reservoir properties, the relative positions of constituent grains must be preserved during coring, handling and analysis. Thus, it is of great importance in core analysis to preserve the inner structure of the core. Uncemented sand samples represent a major challenge regarding handling and analysis.

The main objective of a coring and core preservation program is to obtain the core sample that is representative of the formation and deliver it to the core analysis laboratory as unaltered as possible. A major obstacle to obtaining good core from poorly consolidated (uncemented) reservoir formations has been the lack of a good method which suitably preserves the inner structure of the rock sample. For samples of uncemented sand cores, it is generally required some form of stabilization or preservation inside the core barrels prior to transport.

Several techniques have beed developed for preserving sedimentary structures in uncemented sand samples from drilling holes. Two main stabilization methods are generally used in the industry: freezing of the core and injection of fast hardening epoxy resin/plastics in the core/core barrel annulus. A combination of both methods may also be used. In the Polymer Resin Injection method, the core is stabilized and sealed inside the liner. Thus, it can be handled as more consolidated rock material in pieces of one meter length. These methods show evident disadvantages, thus in freezing core samples technique, due to water expansion during freezing the core material will often be damaged in the process. On the other hand, when a fast hardening epoxy resin is used, formation of secondary products may occur, causing change in the chemical and physical properties of the sample. The resin may affect rock wettability, and wettability would in turn affect most other properties measured in a typical Special Core Analysis (SCAL) programme: i.e. relative permeabilities, capillary pressures and electrical properties.

An alternative method using gypsum for core protection was developed for stabilizing unconsolidated core sample materials. The method comprising encapsulating the core sample material with an encapsulation material supplied to a space established between the core sample material and the pipe; the encapsulation material is a water slurry of calcined gypsum with one or more set-controlling substances, and hardening the resulting plaster slurry to a solid mass which in at least partially encapsulates and stabilizes the unconsolidated core sample material. The method is described in EP832049.

In WO02/04784 it is disclosed a method of consolidating a subterranean formation penetrated by a wellbore or repairing a plugged gravel pack in a wellbore. The method comprises injecting into the subterranean formation a consolidating fluid which comprises a gel component and a gel-forming agent. The gel component is a compound that, under at least some downhole conditions, can form a flexible gel.

Despite the effords in the field, the provision of a method to preserve the inner structure (fabric) of the uncemented sand samples from drilling holes wherein neither the devices nor the samples are liable of damage by the method goes on being an active field of research.

SUMMARY Inventors have surprisingly found that uncemented sand samples from a drilling hole, may be suitably preserved when a polyssacharide solution is injected into the uncemented sand sample which is encapsulated in a sand sample pipe, a rubber sleeve or similar; whereupon the solution is cool down until jellification thus providing sufficient stiffness and protection of the core sample. The resulting protected uncemented sand sample maintain their inner structure, allowing to mobilize the samples to different experimental measuring equipment.

Thus, a first aspect of the present disclosure relates to a method to preserve the inner structure of uncemented sand samples from a drilling hole. The method comprises the injection of a polyssacharide solution into an

uncemented sand sample, which after retrieval from a drilling hole is encapsulated in a sand sample pipe, a rubber sleeve or similar; and then cool down the sample until jellification of the solution thus resulting in a preserved uncemented sand sample. In some examples the injection step is performed in a single step using a polyssacharide solution concentration from 0.7 to 1 .3 % w/w,

In accordance with other examples, the injection step is performed in two or more steps, using a ramp of increasing concentration of the polyssacharide solution until achieving a complete saturation of the sample to the final concentration, comprising a fist injection step wherein a first polysaccharide solution is injected, and at least a second injection step wherein a second polysaccharide solution is injected; wherein:

a) in a first step, it is injected an aqueous solution of the

polysaccharide at a concentration from 0.5 to 1 .5 % w/w; and

b) in a second step, it is injected an aqueous solution of the

polysaccharide at a concentration from 1 .0 to 2.0 % w/w.

In the context of the present disclosure, the term "encapsulate the sample" and variations thereof, refers to confine in or enclose [the sample] in a container.

The term "uncemented sand sample" referes to a sample of sand without detrital cement within their interstices. The term "uncemented sand sample" is also known as "non-consolidated sand sample" in petroleum industry.

The term "detrital cement" refers to carbonated, silicic or ferruginous materials that fill the pore spaces in sedimentary rocks. It precipitates from an ionic or colloidal aqueous solution that flows through the rocks, as a result of the chemical interaction. It reduces the porosity and gives cohesion between clasts and matrix.

The method according to the present disclosure keeps with no disturbance the inner structure (fabric) of oil sand well cores; keeps uncemented sand inner structure after coupled thermo-hydro-mechanical-chemical- core- flooding tests (mechanical-chemical tests); it allows observing inner structure before and after mechanical tests with the aid of complementary techniques; and it allows a successful performance of a set of consecutive mechanical and/or chemical-mechanical tests with the advantage of keeping the resulting inner structure obtained from the preceding test. It is important to note that for the sample preservation according to the method of the present invention, it is unnecessary to maintain the sand sample in a rubber band or a container. Once the sample is injected with the polysaccharide solution and jellification occurs, the preserved sand sample thus obtained can be handed with not container. In accordance with another aspect, the present disclosure relates to an uncemented sand sample whose inner structure has been preserved by the process of the invention which comprises:

a) injection of a polyssacharide solution into a uncemented sample

encapsulated in a sand sample pipe, a rubber sleeve or similar; and b) cool down the sample until jellification of the solution thus resulting in a preserved uncemented sand sample.

In accordance with some examples of this aspect, the injection step is performed in a single step using a polyssacharide solution concentration from 0.7 to 1 .3 % w/w.

In accordance with other examples of this aspect, the injection step is performed in two or more steps, using a ramp of increasing concentration of the polyssacharide solution until achieving a complete saturation of the sample to the final concentration, comprising a fist injection step wherein a first polysaccharide solution is injected, and at least a second injection step wherein a second polysaccharide solution is injected; wherein: a) in a first step, it is injected an aqueous solution of the

polysaccharide at a concentration from 0.5 to 1 .5 % w/w; and

b) in a second step, it is injected an aqueous solution of the

polysaccharide at a concentration from 1 .0 to 2.0 % w/w.

The polysaccharides suitable for the herein disclosed method meet the following characteristics: are soluble in water, jellify during cooling from a threshold temperature, are long-lasting chemically stable, not reactive with sand, are environmental-friendly and non-toxic.

In accordance with an embodiment of the present disclosure, the

polyssacharide solution have a viscosity at the injection temperature from 4- 10 mPas, preferably from 6-8 mPas during the injection. It has been observed that during the injection, may occure any temperature decrease along the pump circuit. The observed temperature decrease must not lead to a significant viscosity increase, which in case of e.g. agar-agar, viscosity increase should not be higher than 1 mPas. Thus, it may be necessary to control the temperature along the pump circuit in order to avoid such a viscosity increasement. Control of pump circuit temperature may be carried out by any means known by the skilled person in the art.

In accordance with a particular embodiment, the polissacharide is selected from agar-agar, agarose, gelrite, xanthan gum, and alginate among others; preferably the polissacharide is agar-agar.

In the context of the present disclosure, the terms "agar" and "agar-agar"refer to the same product, and are considered equivalents. In the natural state, agar occurs as structural carbohydrate in the cell walls of agarophytes algae, probably existing in the form of its calcium salt or a mixture of calcium and magnesium salts. It is a complex mixture of

polysaccharides composed of two major fractions - agarose, a neutral polymer, and agaropectin, a charged, sulfated polymer.

Agarose, the gelling fraction, is a neutral linear molecule essentially free of sulfates, consisting of chains of repeating alternate units of β-1 ,3-linked- D- galactose and a-1 ,4-linked 3,6-anhydro-L-galactose. Agaropectin, the non gelling fraction, is a sulfated polysaccharide (3% to 10% sulfate), composed of agarose and varying percentages of ester sulfate, D-glucuronic acid, and small amounts of pyruvic acid. The proportion of these two polymers varies according to the species of seaweed. Agarose normally represents at least two-thirds of the natural agar-agar.

Agar-agar is insoluble in cold water, but it swells considerably, absorbing as much as twenty times its own weight of water. It dissolves readily in boiling water and sets to a firm gel at concentrations as low as 0.50%. The gelling portion of agar-agar has a double helical structure. Double helices aggregate to form a three-dimensional structure framework which holds the water molecules within the interstices of the framework. Thus, thermo-reversible gels are formed.

Gelling occurs at temperatures far below the gel melting temperature. A 1 .5% solution of agar-agar forms a gel on cooling to about 32° to 45° C that does not melt below 85° C. The viscosity of agar solutions varies widely and is markedly dependent upon the raw material source. The viscosity of an agar solution at temperatures above its gelling point is relatively constant at pH ranging from 4.5 to 9.0, and is not greatly affected by age or ionic strength within the pH range 6.0 to 8.0. However, once gelling starts viscosity at constant temperature increases with time.

In accordance with a particular embodiment, the method is characterized by a process wherein the injection of the aqueous polysaccharide solution is performed in a single step, continuous, process with a unique polysaccharide solution.

In accordance with a embodiment of this particular embodiment, the polyssacharide solution concentration is from 0.7 to 1 .3 % w/w, preferably from 0.8 to 1 .2 % w/w, being particularly preferred 1 .0 % w/w.

In a particularly preferred embodiment, the polysaccharide solution is an aqueous solution of agar-agar comprising agar-agar in a concentration of 1 .0 % w/w, and the process comprises the injection of the polyssacharide solution into a uncemented sample encapsulated in a sand sample pipe, a rubber sleeve or similar in a continuous process, i.e. in a unique injection step. Alternatively, in accordance with another particular embodiment, the method is characterized by a process wherein the injection of the aqueous

polysaccharide solution is performed using a concentration ramp of the polysaccharide solution until achieving a complete saturation of the sample to the final concentration. In accordance with a particular embodiment of this alternative, the injection may be performed in two or more steps, first with a polyssacharide solution concentration and secondly (or in subsequent steps) with the same or different concentration in a concentration ramp; preferably the second and subsequent concentration is higher than the previous concentration. Thus, in accordance with a particularly preferred embodiment, in a first step a polysaccharide solution in a concentration from 0.5 to 1 .5% w/w is injected and in a second step it is injected a second polysaccharide solution in a concentration from 1 .0 to 2.0% w/w, thus resulting in a complete injection without premature gelifying near to the injection zone. In accordance with a particularly preferred embodiment, the polysaccharide solution is an aqueous solution of agar-agar. Thus, in a first step, an aqueous solution of agar-agar at a concentration from 0.5 to 1 .5 % w/w; and in a second step, a second aqueous solution of agar-agar at a concentration from 1 .0 to 2.0 % w/w is injected.

Most preferably, in a first step, and aqueous solution of agar-agar at a concentration of 1 .0% w/w is injected; and in a second step it is injected a second aqueous solution of agar-agar at a concentration of 1 .5% w/w. The injection of the polysaccharide solution may be performed at a

temperature from 60-90 °C, preferably 65-75 °C, being particularly preferred from 70-72 °C.

In accordance with a particular embodiment, the injected volumen of polysaccharide solution into the uncemented sample encapsulated in a sand sample pipe, a rubber sleeve or similar, is from 3 to 8 porous volumen.

Preferably from 4 to 6 porous volumen. When the injection process is carried out using a concentration ramp of the polysaccharide solution, the injected volumen of polysaccharide solution in the first step is from 3 to 8 porous volumen, preferably from 4 to 6 porous volumen; and the injected volumen of polysaccharide solution in the second or subsequent steps is from 3 to 8 porous volumen, preferably from 4 to 6 porous volumen.

Therefore, in accordance with a particular embodiment of the present disclosure, the injection step is performed under the following operation conditions:

- Temperature 60-90 °C, preferably 65-75 °C, being particularly

preferred from 70-72 °C

- First injected volumen: 4 porous volumen at 1 % w/w and a second injected volumen: 4 porous volume at 1 .5 % w/w

In the context of the present disclosure, the term "porous volume" refers to the interconnected pore volume or void space in a rock or packed detritic sample that contributes to fluid flow or permeability. Excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains. Pore volume is calculated by measuring the fluid volume in the sample by a fluid injection.

Fluid injection velocity and solution viscosity have to be established according to the sample permeability and the design of the injection equipment so that no partial gelfication in the sample and/or in the injection lines take place before the whole injection is complete.

Likewise, the agar-agar solution has to be injected at a velocity and with a viscosity such that no particle dragging or inner structure alteration occur.

The polysaccharide solution is a thermoreversible innocuous gel, thus the method herein disclosed is completely reversible. The polyssacharide solution may be easily flushed out from a tested sand sample just by heating back up the sample and injecting hot water. Or it can be simply removed by washing the sample with hot water. Neither the devices nor the samples are liable of (liable of being damage) damage by this method.

The polyssacharide solution may also comprise other components such as biocides, antioxidants, or any other additives which may be neccesaries to preserve and/or provide any suitable properties to the sample.

It has been observed that the method of the present disclosure is particularly suitable for uncemented sand samples having an average grain diameter comprised from 2 to 0.06 mm and a permeability comprised from 10^"1 to 10^"5 cm/s.

Throughout the description and claims the word "comprise" and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word "comprise" encompasses the case of "consisting of. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

EXAMPLES

In this example, a uncemented sand sample was taken from the formation Morichal in Venezuela.

An appropriate amount of polysaccharide (agar-agar) dust was dissolved in distillated water at 90°C in order to obtain the corresponding 0.5%, 1 % and

1 .5% concentration. The resulting aqueous solution exhibits a similar viscosity as water at room conditions.

In a previous step, water was injected into the sample encapsulated into a rubber sleeve until saturation in order to determine the porous volumen of the sample. Four pore volumes of agar-agar solution were injected. The solution injected was maintained at 70 °C while the sample was held inside a rubber sleeve at least at the same temperature. Injection was made at a fluid injection velocity of 2 cm³/min and maintaining the viscosity of the sample between 6-8 mPas. Once the injection was complete, the sample was cool down until the solution temperature was below 40 °C (the threshold for subsequent jellification). From this moment on, the formed gel could maintain the inner sample structure.

Three different polysaccharide solutions (concentration of agar-agar: 0.5, 1 and 1 .5%), were injected into uniform sand samples (0.25 mm average grain size) in a continuous process.

The results are illustrated below:

A. 0.5% w/w agar-agar concentration: The sample obtained exhibited a low consistency. The sample could hardly maintain the sample shape.

B. 1 % w/w agar-agar concentration: The sample obtained exhibited a high consistency.

C. 1 .5 % w/w agar-agar concentration: The injection failed. The injection front within the first millimeters of the sand sample starts to jellify preventing further flow.

Special care with temperature drop during injection should be taken. In the example, injection lines were isolated and warmed using an electrical resistance.

In a fourth example (D), the polysaccharide injection was performed using a concentration ramp in two stages:

D. Two injection stages:

i) injection with a solutoin of agar-agar 1 % w/w maintaining the process temperature at 70 °C, injection of a volume equivalent to 4 pore volumes was required;

ii) in a second stage, it was injected a solultion of agar-agar at 1 .5% w/w concentration maintaining the process temperature at 70 °C, injection of a volume equivalent to 4 pore volumes was required. Full injection were achieved avoiding premature gelifying near the injection zone.

The sample exhibited a high consistency. When the soil inner structure is not needed any longer, the jellified

polysaccharide can be easily removed from the sample just by a wash. In the present example, sample was washed with hot water at 70 °C until the effluent flow was free from polysaccharide in solution. After washing, no traces of polysaccharide (agar) remained on the grain surfaces.

REFERENCES CITED IN THE APPLICATION 1 . EP832049

2. WO02/04784

Claims

1 . A method to preserve the inner structure of uncemented sand samples from a drilling hole, the method comprising

a) injection of a polyssacharide solution into an uncemented sand sample, which after retrieval from a drilling hole is encapsulated in a sand sample pipe, a rubber sleeve or similar; and

b) cool down the sample until jellification of the solution thus resulting in a preserved uncemented sand sample;

wherein

i) the injection step is performed in a single step using a polyssacharide solution concentration from 0.7 to 1 .3 % w/w:

or alternatively

ii) the injection step is performed in two or more steps, using a ramp of increasing concentration of the polyssacharide solution until achieving a complete saturation of the sample to the final concentration, comprising a fist injection step wherein a first polysaccharide solution is injected, an at least a second injection step wherein a second polysaccharide solution is injected;

wherein

a) in a first step, it is injected an aqueous solution of the polysaccharide at a concentration from 0.5 to 1 .5 % w/w; and b) in a second step, it is injected an aqueous solution of the polysaccharide at a concentration from 1 .0 to 2.0 % w/w.

2. The method according to claim 1 , wherein the injection of the

polysaccharide solution is performed at a temperature from 60-90 °C.

3. The method according to any of claims 1 -2, wherein the polissacharide is selected from agar-agar, agarose, gelrite, xanthan gum, and alginate.

4. The method according to any of claims 1 -3, wherein the polysaccharide solution is agar-agar.

5. The method according to any of claims 1 -4, wherein the injection of the aqueous polysaccharide solution is performed in at least two steps, using a concentration ramp of the polysaccharide solution until achieving a complete saturation of the sample to the final concentration; comprising a fist injection step wherein a first polysaccharide solution is injected, an at least a second injection step wherein a second polysaccharide solution is injected.

6. The method according to any of claims 1 -5, wherein the injection of the aqueous polysaccharide solution is performed in two steps:

i) in a first step, it is injected an aqueous solution of the polysaccharide at a concentration from 0.5 to 1 .5 % w/w; and

ii) in a second step, it is injected an aqueous solution of the

polysaccharide at a concentration from 1 .0 to 2.0 % w/w.

7. The method according to any of claims 1 -6, wherein the injection of the aqueous polysaccharide solution is performed in two steps:

i) in a first step, it is injected an aqueous solution of the polysaccharide at a concentration from of 1 .0 % w/w; and

ii) in a second step, it is injected an aqueous solution of the

polysaccharide at a concentration of 1 .5 % w/w.

8. The method according to any of claims 1 -7, wherein the injection step is performed in two steps, while maintained the temperature from 60-90 °C; and wherein in a first step there are injected 4 porous volumen of an agar-agar solution 1 % w/w; and in a second step there are injected 4 porous volumen of an agar-agar solution 1 .5 % w/w.

9. The method according to any of claims 1 -4, wherein the injection of the aqueous polysaccharide solution is performed in a continuous process with a polysaccharide solution at a concentration from 0.7 to 1 .3% w/w.

10. The method according to any of claims 1 -9, wherein the uncemented sand sample has a average grain diameter comprised from 2-0.06 mm and a permeability comprised from 10^"1 to 10^"5 cm/s.

1 1 . A preserved uncemented sand sample from a drilling hole whose inner structure has been preserved by the process according to any of claims 1 -10.

12 The preserved uncemented sand sample according to claim 1 1 wherein the polyssacharide solution is an aqueous solution of agar-agar.