Gas Gravity Drainage in Fractured Reservoirs Through New Dual-Continuum Approach (includes associated papers 20296 and 20390 )

Abstract

Summary Present theory assumes that a capillary holdup zone of oil remains in all single matrix blocks in fractured reservoirs producing by gas gravity drainage. Model simulations applying a dual-continuum approach in a conventional reservoir simulator showed that even with a low degree of continuity through the matrix between the blocks, the oil from the capillary holdup zone in each block is produced. The expected oil production rate and recovery is much higher than predicted by the present theory. Introduction Production mechanisms-such as expansion, solution-gas drive. oil/water imbibition, and gas gravity drainage-may all contribute to production of oil at different stages in the production life of a fractured reservoir. Gas gravity drainage takes place when gas from the gas-saturated fractures displaces oil in the matrix. The free gas may be gas liberated from the oil that has segregated in the fractures, gas from an expanding gas cap, or gas from gas injection. The density difference between the oil and the gas phases provides the energy for the gas-gravity-drainage process. Both gas gravity drainage and oil/water imbibition act without depleting the reservoir. The latter has received more attention as a potential production mechanism in literature and research during the last 2 decades. These production mechanisms are at present evaluated on single, vertical, regular matrix blocks. A capillary holdup zone remains in each single block after the gas-gravity-drainage process has ceased. The influence of capillary pressure reduces both oil production rate and oil recovery. The gas-gravity-drainage process in stacks of two blocks, where the blocks are in capillary continuity through the matrix, is analyzed in this paper with a dual-continuum approach in a conventional reservoir simulator. The results show that models of single isolated blocks underestimate the expected oil production rate and oil recovery. Theory Gas gravity drainage is the essential production mechanism when liberated solution gas or injected gas invades the fractures and displaces oil from the matrix. Production mechanisms-such as expansion, oil/water imbibition, and gas gravity drainage-are usually evaluated with a simplified model of the matrix represented by single blocks. Single Block. Free gas invades the fractures surrounding the oil-saturated matrix blocks. Oil starts to drain downward because of the gravity difference between gas and oil, while gas enters at the top of the block to replace the produced oil. The permeability of the fractures surrounding a block is infinite compared with the permeability of the matrix. and gas pressure is assumed to follow the static gas gradient. Eq. 1 describes the oil flow rate vertically through a cross section of the block (see the Appendix): (1) Oil flow rate is expressed as a function of capillary pressure and relative permeability, which are assumed functions of saturation only. No general analytical solution is available even for the highly simplified one-dimensional flow in a single block. Some conclusions about oil production rate and recovery can be drawn from the known initial conditions. Eq. 2 gives the initial capillary pressure gradient (see the Appendix): (2) This is illustrated in Fig. 1 for two different block heights. Eq. 3 describes the initial oil production rate from the block: (3) The oil production rate decreases rapidly from its initial maximum value as both the relative permeability for oil and the capillary pressure gradient decrease with the increasing gas saturation in the block. Gas cannot enter the matrix block until the gas/oil pressure difference exceeds the capillary entrance pressure. The required pressure difference is equivalent to a threshold height: (4) The relationship between block height and capillary threshold height controls the oil production rate from the block. When the block height is large compared with the capillary threshold height, the influence of capillary pressure on rate in Eq. 3 is negligible: (5) When block height is near the capillary threshold height, the gas/oil pressure difference is less than the capillary entrance pressure over most of the block (Eq. 6). Low rate and poor recovery are to be expected, and gas gravity drainage is not considered a viable production mechanism. (6) Stack of Blocks. The stack-of-blocks concept was introduced when it was realized that oil produced from the base of one block would reinfiltrate into the block below. The gas-invaded zone is represented by a number of single blocks stacked on each other through which oil drains from block to block down to the gas/oil contact (GOC). Because oil flows through the matrix, the expected rate of oil production is lower than when oil drains from each single block and flows through the fractures to the wells. Tortuous Continuous Matrix. The performance of heavily fractured oil reservoirs producing below their bubblepoint pressures cannot be matched by the single-block or the stack-of-block models. Sustained high oil production rates. low rates of pressure decline, and large volumes of gas remaining in the reservoir have been observed. The single-block model underestimates production rates and oil recovery. The single-block concept is valid only if the matrix is completely cut by a three-dimensional fracture network. Fractures presented in two dimensions give the impression that only three or four fractures are sufficient to create a block. This is not the case in three dimensions; e.g., a cross section through a spiral telephone cord will show a large number of "blocks"instead of a continuous cord. The matrix can thus be expected to be more continuous in the reservoir than it may appear from core observations. Also. it can be observed that even if a surface outcrop is heavily fractured, it is often not possible to pick out individual blocks on the rock surface. These observations suggest that even if the reservoir is cut by numerous fractures, the matrix rock may still be connected. The matrix may be imagined as tortuously continuous. This feature can be modeled by a stack of matrix blocks separated by fractures. as before, but with additional connections through the matrix over a limited cross-sectional area between the blocks. SPERE P. 271^