Wells – Processes – Distinct – separate injection and producing wells
Reexamination Certificate
1998-12-15
2001-02-20
Suchfield, George (Department: 3672)
Wells
Processes
Distinct, separate injection and producing wells
C166S295000, C166S300000, C507S225000, C507S267000, C507S271000, C507S903000, C523S130000
Reexamination Certificate
active
06189615
ABSTRACT:
TECHNICAL FIELD
The invention relates to a process for recovering hydrocarbons from a subterranean formation, and more particularly to treatment of an alkaline region with a stabilized permeability or fluid mobility reducing agent, thereby improving hydrocarbon recovery from the subterranean formation.
BACKGROUND OF THE INVENTION
The utility of a crosslinked polymer gel as a permeability reducing agent in or near a subterranean hydrocarbon-bearing formation to facilitate hydrocarbon recovery from the formation has long been known in the art. U.S. Pat. No. 3,762,476 to Gall is representative of conventional teaching in the art of its time. Gall discloses a conformance improvement treatment process, wherein a permeability reducing crosslinked polymer gel is formed in situ by injecting an aqueous slug containing a crosslinkable water-soluble polymer into a subterranean treatment region followed in sequence by an aqueous slug containing a crosslinking agent including a polyvalent metal cation. Useful polyvalent metal cations listed in Gall include iron(II), iron(III), aluminum(III), chromium(III), calcium(II), and magnesium(II).
Sequential injection of the gel components as separate and distinct slugs into the treatment region is essential to the teaching of treatment processes such as Gall insofar as surface contacting of the polymer and crosslinking agent was believed to cause premature gelation of the gel components prior to reaching the treatment region. If prematurely formed at the surface or in the well bore, placement in the treatment region of the gels taught by Gall was difficult, if not impossible, to achieve. Consequently, sequential injection of the gel components in concept avoided premature gelation by delaying contacting of the components until they were displaced out into the treatment region. In situ contacting of the polymer and crosslinking agent as required by Gall, nevertheless, proved operationally unattractive in many hydrocarbon recovery applications because of the difficulty in achieving adequate mixing of the gel components in situ. Without adequate mixing, the gels of Gall were poorly formed, resulting in weak and unstable gels which performed ineffectively as permeability reducing agents.
In response to the shortcomings of sequential injection processes such as Gall, U.S. Pat. No. 4,683,949 to Sydansk et al identified specific gel components and gelation parameters for a crosslinked polymer gel having utility in a conformance improvement treatment process, whereby the polymer and crosslinking agent of the gel could be mixed at the surface in a homogeneous gelation solution and placed in the treatment region by injection therein as a single slug. Thus, Sydansk et al overcame the inherent operational limitations of processes such as Gall which required sequential injection and in situ mixing of the gel components. The gel technology of Sydansk et al was predicated on the finding that a chromium(III)/carboxylate complex crosslinking agent could be mixed with a crosslinkable polymer at the surface to form a gelation solution which produced a uniquely stable, highly predictable and beneficially performing gel in situ.
Although the process taught by Sydansk et al is generally effective for most treatment applications, certain treatment applications have proven problematic. For example, high temperature regions having a substantial concentration of carbonate minerals in the rock matrix are a difficult environment for treatment. The carbonate minerals partially dissolve into the formation waters of the high temperature treatment region and tend to damage the gel or gelation solution on contact. The carbonate minerals form alkaline species in solution, attacking or interfering with the chromium(III) which is present in the crosslinking agent. The alkaline species tend to convert chromium(III) to chromic hydroxide precipitate, depleting the availability of chromium(III) for polymer crosslinking. Consequently, the stability and effectiveness of the resulting gel as a permeability reducing agent can be significantly diminished. As such, a need exists for a process employing a crosslinked polymer gel as a permeability or fluid mobility reducing agent, wherein the gel has improved stability and effectiveness in alkaline environments, and particularly in high temperature carbonate treatment regions.
Accordingly, it is an object of the present invention to provide a process for applying a stabilized crosslinked polymer gel to a subterranean region which is highly alkaline. It is more particularly an object of the present invention to reduce the permeability of or fluid mobility within the alkaline treatment region to which the stabilized gel is applied. It is also an object of the present invention to improve the recovery of hydrocarbons from a hydrocarbon-bearing formation which contains alkaline treatment region or which is in fluid communication with the alkaline treatment region. More particularly, it is an object of the present invention to improve the recovery of hydrocarbons from a hydrocarbon-bearing formation which contains a carbonate treatment region or which is in fluid communication with a carbonate treatment region. It is another object of the present invention to improve the recovery of hydrocarbons from a hydrocarbon-bearing formation which contains a high temperature treatment region or which is in fluid communication with the high temperature treatment region. These objects and others are achieved in accordance with the invention described hereafter.
SUMMARY OF THE INVENTION
The present invention is a process for substantially reducing the permeability of a treatment region in or proximate to a hydrocarbon-bearing formation below an earthen surface penetrated by a well bore. Alternatively, the invention is a process for substantially reducing the fluid mobility within the treatment region. The process is generally applicable to a treatment region exhibiting a substantial degree of alkalinity. The process is particularly applicable to a treatment region containing a substantial concentration of carbonate minerals in the matrix of the treatment region. Such matrix is either a carbonate rock, or a sandstone rock having a substantial content of carbonate sand-grain cementation minerals. The process is more particularly applicable to carbonate-containing treatment regions exhibiting a high temperature. The process is still more particularly applicable to carbonate-containing, high temperature treatment regions which exhibit high permeability due to fractures.
The process is initiated by preparing a gelation solution at the earthen surface. The gelation solution is an admixture comprising at least one of each of the following components: an acrylamide polymer, a crosslinking agent, a stabilizing agent, and an aqueous solvent; optionally one or more modifying agents may also be added. The stabilizing agent is selected from a group consisting of acid additives, acidic pH buffer systems, delayed acid generators, and mixtures thereof. An acid additive having utility herein is a strong acid such as hydrochloric acid or sulfuric acid, or preferably a weak acid such as acetic acid. An acidic pH buffer system having utility herein is an acid/base pair such as an acetic acid/sodium acetate pair. A delayed acid generator having utility herein is an ester or anhydride such as ethyl formate ester or acetic anhydride.
The crosslinking agent is a chromium(III) complex having one or more chromium(III) cations and one or more carboxylate anions selected from the group consisting of mono-carboxylates, poly-carboxylates, substituted derivatives of mono- and poly-carboxylates, and mixtures thereof. The carboxylate anion is preferably selected from the group consisting of acetate, propionate, lactate, glycolate, malonate and mixtures thereof.
One or more additional modifying agents can optionally be included in the gelation solution which modify the properties of the gelation solution, the gelation reaction, or the resulting gel. Modifying agents include density control agent
Ebel Jack
Marathon Oil Company
Suchfield George
LandOfFree
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