Improved Polyacrylamide Treatments for Water Control in Producing Wells

Abstract

Summary Two new polyacrylamide processes for water control in producing wells improve the efficiency of conventional polyacrylamide treatment withoutinducing any risk of well plugging by crosslinkers. Treatment plugging bycrosslinkers. Treatment of a gas-storage well strongly decreased waterproduction without any adverse effect on gas injection or production for atleast 3 years. Introduction Excessive water production is frequently encountered during the lifetime ofoil- or gas-producing wells. It is generally responsible for both a rapidproductivity decline and an increase in operating costs caused by the handlingand treating of large amounts of water. It may also induce some secondaryproblems, such as fines or sand migration problems, such as fines or sandmigration in the near-wellbore portion of the reservoir, that also contributeto the deterioration of well productivity. The result often is a prematureshut-in of the well because premature shut-in of the well because productionhas become uneconomical. production has become uneconomical. Direct treatmentsof producing wells to reduce water production may be classified in two maincategories, each one better suited to specific conditions. If water andhydrocarbon zones are clearly separated, processes using permanent barriers, which can be selectively placed in the water zone, generally give good results. This impermeable barrier, which aims to stop the flow of all fluids, may beformed by cements, resins, or silicate gels. On the other hand, when oil or gasand water zones are not easily distinguishable, the use of permanent plugsbecomes very risky. It is generally preferable to place a selective barrier inthe whole pay zone, which blocks water but remains permeable to oil or gas. Inthis case, the systems used are based on hydrosoluble polymers. It has been shown that, after injection of high-molecular-weighthydrosoluble polymers in the near-wellbore region of a polymers in thenear-wellbore region of a reservoir, the polymer adsorbed on the rock has theproperty to restrain water flow with little effect on oil or gas flow. Moreover, polymer adsorption appears to be almost polymer adsorption appears tobe almost irreversible, thus making the process efficient for long periods oftime. The polymers most widely used in this process are polyacrylamides havinga certain percentage polyacrylamides having a certain percentage (generallyless than 30%) of acrylate functions. These polymers, readily available aspowders or emulsions, are relatively cheap. powders or emulsions, arerelatively cheap. They have a high viscosifying power, especially in softwater, and give high values of permeability reduction to brine when they areabsorbed on reservoir rocks. Conventional processes are based on the adsorption properties of singlehigh-molecuiar-weight polyacrylamides. Adsorbed polymolecules form an almostimpenetrable polymolecules form an almost impenetrable layer on pore walls, thereby effectively restraining the flow of water (the wetting phase) while thecenter of the pore channels phase) while the center of the pore channelsremains free for the flow of hydrocarbons (the nonwetting phase) Fig. 1a is aschematic of this process. The efficiency of the process is directly related tothe thickness of the process is directly related to the thickness of theadsorbed layer compared to pore size. In high-permeability or fissuredreservoirs, the channels through which water flows are an order of magnitudelarger than the macromolecular size, rendering the process quite inefficient. To treat such cases, several processes using crosslinking agents have beenprocesses using crosslinking agents have been developed. Crosslinkers aregenerally multivalent metal ions, which can build polymer networks in the porechannels (Fig. 1b). Although such treatments provide a greater ability to blockwater, they also carry the risk of losing some selectivity, indeed, because thecenter of the pore channels is occupied by the polymer network, the flow of thenonwetting hydrocarbon phase becomes restrained in the same way as that ofwater. Two new polyacrylamide-based treatments improve the efficiency ofconventional processes without the risk of well impairments associated withcrosslinked systems. Both are based on the in-situ swelling of the adsorbedpolymer layer. Process A is very efficient for treating reservoirs havinglow-salinity brines; Process B covers a greater range of salinity. Both processes have been evaluated in the laboratory through two-phase flowtests in sand and sandstone cores. We could thus determine the changes in waterand oil or gas relative permeabilities induced by the treatments. The swellingof the adsorbed polymer layer was followed by the measurement polymer layer wasfollowed by the measurement of permeability reductions to brine before andafter treatment. Process A was applied to a gas-storage well in June 1986. See the Cerviile-Velaine Field Test section for results of this test. Process Description Process Description Process A. Process A uses theproperties Process A. Process A uses the properties of the acrylamide/acrylatecopolymer's (HPAM) coiled molecule that cause it to shrink in the presence ofsalts and to swell in soft water (Fig. 2). JPT P. 862