Liberation of Solution Gas During Pressure Depletion of Virgin and Watered-Out Oil Reservoirs

Abstract

Summary Multipurpose experimental equipment was constructed to investigate thebuildup of gas saturation during depressurization of virgin and watered-out oilreservoirs at representative conditions. The measured dependencies of criticalgas saturations on gas/oil interfacial tension (IFT), amount of dissolved gas, pressure-decline rate, and structure of the porous medium are discussed. Inaddition, experimental results indicating porous medium are discussed. Inaddition, experimental results indicating significant reductions of waterfloodresidual oil saturation (ROS) owing to the presence of a gas saturation arepresented. Introduction When pressure drops below the bubblepoint pressure in virgin or watered-outoil reservoirs, gas comes out of solution and a gas saturation builds up. Theprocess can be divided into three stages:the formation of gas bubbles,the growth of gas bubbles, andthe upward migration of part of theliberated gas, which creates a network of gas channels. Knowledge of the buildup of a gas saturation and subsequent migration is essential in reservoir development planning. These events will affect recovery in a solution-gas-driveprocess' and in a water-injection project, for which the timing and the optimumpressure level depend on the value of the critical gas saturation and pressurelevel depend on the value of the critical gas saturation and the waterflood ROSin the presence of a built-up gas saturation. In addition, formation of gasbubbles inside oil globules during blowdown of a watered-out oil reservoir orexpansion of residual gas as a watered-out gas reservoir is depleted may leadto remobilization of trapped hydrocarbons. The formation of a new gas/liquidinterface requires energy. Therefore, the liquid has to be supersaturatedi.e., at a pressure lower than the bubblepoint pressure. Once a bubble has formed, gas from the surrounding liquid will diffuse toward that bubble. This reducesthe degree of supersaturation and, as a result, prevents formation of newbubbles close to the existing bubble(s). Diffusion is slow, however, and thecontinued pressure decline may cause a degree of supersaturation at somedistance from the existing bubble(s) such that more bubbles are formed. Thesecond stage of the solution-gas drive, the growth of gas bubbles resultingfrom expansion and diffusion, therefore partly overlaps the first stage ofbubble formation. In the third stage, upon continued pressure decline, gasbubbles grow to such an extent that they break pressure decline, gas bubblesgrow to such an extent that they break through the pore throats and form gaschannels. Buoyancy forces promote growth in the vertical directioni.e., upwardmigration promote growth in the vertical directioni.e., upward migration ofgas. The shape of the gas channels depends on the difference between thecapillary pressures of the various pore throats. With small differences, thegas can leave the pore through several Pore throats, resulting in a branch ofchannels (dispersion). With large differences in capillary pressures, the gaschannel will grow along the route with the lowest capillary resistance(nondispersion). The gas saturation will increase until all gas channels havereached the top of the liquid column and, in the case of dispersion conditions, until the separate gas channels are interconnected. Expanding and newlyliberated gas can then flow freely to the top of the reservoir. The saturationat which this occurs is the critical gas saturation. The buildup of gassaturation upon pressure decline is a very complicated process. The criticalgas saturation will depend on a large number of parameters. Important andpossibly controlling parameters are the gas/liquid IFT, the amount of dissolvedgas and parameters are the gas/liquid IFT, the amount of dissolved gas and itspressure dependence, the pressure less than decline rate, and the structure ofthe porous medium. Previous studies on solution-gas drive, aimed at derivinggas and oil relative permeabilities, indicated a strong dependence onpressure-decline rate. Those studies, however, concentrated on thepressure-decline rate. Those studies, however, concentrated on the mechanism ofparallel flow of oil and gas and neglected the influence of gravity. Moreover, the IFT's were not representative of reservoir conditions. The objectives ofthis study were (1) to assess the influence of the abovementioned parameters onthe critical gas saturation, (2) to estimate critical gas saturations at fieldpressure-decline rates for two North Sea reservoirs, and (3) to assess possiblereductions of waterflood ROS's caused by the presence of a gas saturation. Experimental Equipment and Procedures Equipment. Experimental equipment was designed and constructed at Koninklijke/Shell E and P Laboratorium (KSEPL) specifically to investigate howthe buildup of gas saturation depends on the abovementioned parameters atelevated pressures and temperatures. The equipment enables solution-gas-driveexperiments at representative gas/oil IFT's on both virgin and watered-outcores. In addition, it can be used to simulate the follow-up of pressuredepletion by a waterdrive and the sequential invasion of oil and water into thegas capi.e., to determine the reductions of waterflood ROS caused by thepresence of liberated solution gas or residual gas. Fig. 1 is a flow diagram ofthe equipment. The main parts are a Hassler-type core holder, liquid supplyvessels, production vessels, a circulation and injection pump, amicroprocessor-controlled pressure-regulation system, and vessels in which theshrinkage of pressure-regulation system, and vessels in which the shrinkage ofthe liquid can be determined for conditions of supersaturation andthermodynamic equilibrium. In addition, gamma ray sources at two energy levelsand a gamma ray detector are mounted for in-situ saturation measurements. Allthese parts are housed in a thermostatic cabinet, the temperature of which iscontrolled within 0.18 degsF [0.1 degC]. Maximum operating temperature is 113degsF [45 degC] and maximum pressure 1,450 psi [10 MPa]. The system pressureduring the various stages of an experiment is regulated by a moving piston in alarge cylinder that forms part of the system. At a piston in a large cylinderthat forms part of the system. At a system pressure higher than a prescribedvalue, the cylinder volume is enlarged and vice versa. The cylinder is locatedinside the thermostatic cabinet with a mechanism to move the piston. Thestepping motor required to rotate the spindles, and hence to move the piston, is coupled to a microprocessor, which compares measured piston, is coupled to amicroprocessor, which compares measured and prescribed values and ensures asmooth restoration to the prescribed pressure. prescribed pressure. The systempressure is determined by measuring the eigenfrequency of a quartz crystal. Thepressure-regulating system can maintain a pressure of 1,450 psi [10 MPa] towithin 0.44 psi [3 kPa]. In pressure of 1,450 psi [10 MPa] to within 0.44 psi[3 kPa]. In addition, any pressure decline rate between 0.44 and 14.5 psi/hr [3and 100 kPa/h] can be imposed. The solution-gas-drive experiments are carriedout on cleaned, water-wet cores with a typical diameter of 2 in. [5 cm] and amaximum length of 6 in. [15 cm]. A pressure-addition system exerts a pressure145 psi [1 MPa] above the system pressure on the rubber sleeve surrounding thecore and thereby prevents bypassing problems. problems.