Effect of heterogeneity on the productivity of vertical, deviated and horizontal wells in water drive gas reservoirs

• Heterogeneity effect on performance of water drive gas reservoirs was studied.
• Design of experiments and response surface methodology was employed.
• Recovery and water production is considered to study the heterogeneity effect.
• Layers permeability distribution controls the gas and water production.
• Recovery increases by 8.6–16.2% through increasing heterogeneity level.

Gas recovery from water drive gas reservoirs depends on many engineering and geologic factors. Type of well completion, reservoir heterogeneity and layering pattern are typical factors which determine the ultimate gas recovery. This paper presents the effect of heterogeneity on performance of water drive gas reservoirs by considering recovery factor and cumulative water production. In this regard, Design of Experiments (DOE) and Response Surface Methodology (RSM) are used to investigate the simultaneous effect of effective parameters using a reservoir simulation model. Box–Behnken design (BBD) is applied to implement reservoir simulator. Factors that have been inspected include well inclination angle (Ө), layers permeability distribution (LPD), geometric mean of reservoir horizontal permeability (KG), permeability anisotropy (Kv/Kh), production rate (Qg) and level of heterogeneity in terms of Dykstra–Parsons coefficient (VDP). Three different permeability distribution of increasing downward, random and increasing upward have been considered. In simulation tests with systematic distribution of increasing upward, the permeability of layers increases within lower part towards the upper layer. However in cases with systematic distribution of increasing downward, the layers permeability increases from upper to the bottom layer of the model. Also in random distribution, the layers have random values of permeability.Results indicate that distribution of layers permeability controls the effect of other factors on water drive gas reservoirs performance. For random and systematic distribution of increasing upward, three factors namely geometric mean of reservoir horizontal permeability, permeability anisotropy and level of heterogeneity play an important role on gas productivity. In terms of contribution to total variance, level of heterogeneity for random and increasing upward distribution cases has effect of 3.7% and 41.7% respectively. Increasing level of heterogeneity increases recovery factor by 8.6%–16.2% for the base case. However, in the case of systematic distribution of increasing downward, the level of heterogeneity has very weak effect on objective functions and, could be truly ignored during reservoir performance predictions.