Methods for deposition of materials in underground reservoirs

Details Methods for deposition of materials in underground reservoirs Methods for deposition of materials in underground reservoirs

2000-04-28

2002-06-11

Bagnell, David (Department: 3672)

Wells

Processes

Chemical inter-reaction of two or more introduced materials

C166S270000, C166S295000, C166S304000

Reexamination Certificate

active

06401819

ABSTRACT:

The method of this invention is generally applicable to the control of fluid movement in underground reservoirs through the reduction of the porosity or permeability of the geological formation. The method is especially suitable for use in the recovery of oil and gas from hydrocarbon containing reservoirs.
During oil production operations, a range of problems are encountered arising from the unwanted breakthrough of an overlying gas body, or an edge or bottom water, to the production well by coning or channelling. This is a particular problem where reservoir heterogeneities such as fractures or high permeability streaks are selectively depleted of oil, allowing the premature entry of adjacent gas or water into the production zone. In heavy oil reservoirs, channelling or fingering of water through the relatively immobile oil phase can result in loss of heavy oil production.
A range of methods have been employed in order to increase the recovery of oil from underground reservoirs. In one form of enhanced recovery, a drive fluid is injected under pressure into the oil reservoir through one or more injection wells to maintain, restore or produce formation pressure. The most widely used drive fluid is water. More complex aqueous systems, such as those containing polymer or surfactant, or other fluids such as solvents or gases may also be used. Steam may be used for heavy oils. The drive fluid is often introduced into the oil-bearing underground formation near the bottom of the formation at or above reservoir pressure, to displace oil in the formation. As the fluid moves through the reservoir, it drives or flushes the oil through the formation. An increasing oil saturation develops ahead of the moving fluid and finally reaches the production well or wells. Generally, an oil-bearing underground formation will consist of various regions having different permeabilities. Drive fluid moves preferentially through the regions of higher permeability and in so doing, bypasses oil contained in much lower permeability regions. This obviously reduces the sweep efficiency of the displacing medium.
The flow of fluids through the formation may be modified to improve the production of oil. Reducing the permeability of selected regions can reduce coning, channelling or fingering or improve the sweep efficiency during primary, secondary or enhanced production.
A number of approaches have been proposed to reduce permeability. Processes which use crosslinked polymers or other types of gels have been most common. Other processes using foams, emulsions, suspended solids, microorganisms and precipitates have also been proposed (Seright, R. S. & Liang, J.; Paper SPE 30120 A Comparison of Different Types of Blocking Agents. pp. 431-440 In Proceedings of the European Formation Damage Control Conference, May 15-16, 1995, The Hague, The Netherlands). A number of these processes use hazardous chemicals. Thermal or bacterial degradation of the blocking agent may occur.
The precipitation or deposition of materials within the formation may arise from mixing two or more incompatible chemical solutions in the formation or selectively removing a chemical or chemicals which keep other chemicals in solution. If the process occurs rapidly, however, placement of the precipitate can be difficult.
Ferris and Stehmeier (U.S. Pat. No. 5,143,155) teach that bacteria may be used to precipitate minerals from an aqueous system. Growth of the bacteria on nutrients is required before the minerals are precipitated, allowing some time to place the fluid. However, bacterial systems suffer from a number of potential disadvantages. Nutrients must be supplied. These may be used by organisms other than the intended species or strains, either introduced or indigenous. The bacteria must grow under the reservoir conditions of temperature, pH and salinity. These are often sub-optimal for the preferred organisms. The efficiency of conversion of growth nutrients to desirable products is often low. Bacteria may produce different metabolic products to those intended. The degree of control over the system, including the rate at which precipitation occurs is limited. In addition, bacteria may not readily enter anything other than a high permeability formation due to their size.
Acidising of underground reservoirs using a combination of esterase or lipase enzymes and esters has already been described (PCT/GB94/00922, PCT/GB95/01295). The use of the produced acid to precipitate or deposit other chemicals was not taught.
The present invention teaches the use of enzymes to precipitate or deposit materials within an underground reservoir. Preferably the underground reservoir is a hydrocarbon, for example gas or oil, or water reservoir. The method of precipitating or depositing chemicals within underground reservoirs comprises introducing into the reservoir in aqueous solution (i) an enzyme and (ii) a substrate for said enzyme, such that the action of the enzyme on the substrate leads to the precipitation or deposition of material within the underground reservoir.
The material which is precipitated or deposited may be present, in whole or in part, in the reservoir before the introduction of the enzyme and the substrate.
Alternatively, the material is precipitated or deposited from an aqueous solution or dispersion (iii) introduced into the reservoir in addition to the enzyme and substrate. It is preferable but not essential to use an aqueous solution or dispersion (iii).
It is necessary to select an enzyme which remains active under reservoir conditions. The following parameters are generally taken into consideration:
1) Temperature Tolerance
The temperature of a reservoir is a function of its depth and can be in excess of 100° C. Many onshore reservoirs and some offshore reservoirs in carbonate formations are fairly shallow with temperatures falling within the 30-60° C. range. Generally the enzymes used in the method of the present invention are active between 15° C. and 110° C., for example between 15° C. and 95° C. but an enzyme which is active at higher temperatures may also be used. The enzymes used in the process of the invention have a range of temperatures over which they are active. When there is a temperature gradient in the oil/gas well, it may be desirable to use two or more enzymes together to ensure reliable operation over the temperature range within the well.
2) Pressure Tolerance
Pressure is also a function of depth. Pressures in offshore reservoirs in, for example, the North Sea may exceed 500 atmospheres, whereas shallower on-shore fields are likely to be in the range 50-150 atmospheres. If enzymes are to be injected at rates above fracture pressure, they must withstand injection pressures which will exceed reservoir pressure.
3) Salt Tolerance
The ability to withstand high salt levels is important as reservoir brines can often be near saturated solutions. Enzymes may be injected in fresh water, but they will need to withstand the effects of salts diffusing into that fresh water.
4) Oil Tolerance
Enzymes must be tolerant of oil although they may remain in the aqueous phase within the reservoir.
The enzyme used in the method of the present invention is generally a water soluble enzymrte. It is advantageous for the enzyme to be readily water soluble. Preferably the enzyme is a hydrolase (EC 3) such as a lipase (EC 3.1.1.3), an esterase (EC 3.1.1.1) or a urease (EC 3.5.1.5) or an oxidoreductase (EC 1.) such as an oxidase or peroxidase.
Typically isolated enzymes are used. Enzymes may be isolated from plant, animal, bacterial or fungal sources. The enzymes may be produced from wild-type, conventionally bred, mutated or genetically engineered organisms. The enzymes may, optionally, be chemically modified, as long as they retain or possess a desired catalytic ability. Individual enzymes are selected for their ability to act on the selected substrate, producing a desired change under the conditions of the underground reservoir. Preferably, the enzymes will be industrial enzymes available in bulk from commercial sources.
The substrate is gen

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