Analysis of production prediction in shale reservoirs: Influence of water film in inorganic matter

Abstract

To commercially develop shale gas reservoirs, multi-fractured horizontal wells are widely employed in the industry. A number of analytical models have been proposed to evaluate and forecast production from fractured horizontal wells. However, the water film adsorbed in inorganic pores of shale matrix is often neglected and its effect on gas production has never been focused on. Moreover, the effect on gas production caused by the characteristic of a well not located in the center of drainage area has not been drawn much attention. In this work, on a basis of enhanced-fracture-region (EFR) model, a new model is put forward to conduct production prediction of multi-fractured horizontal wells from shale gas reservoirs. This novel approach incorporates critical gas transport mechanisms in shale, and considers the characteristic of a well not located in the center of drainage area. Specifically, this model takes the bulk-gas transport regimes, water film effect, stress dependence and real gas effect into account, which matches well with the real shale reservoirs. The analytical solution of the proposed model is deduced by employing the Laplace transformation approach, and then introducing the numerical algorithm put forward by Stehfest to invert to the real time domain. This presented model is verified by both numerical simulation cases and actual field applications. In addition, a sensitivity study is performed to illustrate various parameters on flow-regime and production curves. Results illustrate that both the size of stimulated zone and distance from the well to the outer boundary contribute to the well performance. A larger size of the stimulated zone increases the duration of the first linear flow period, whereas will shorten the duration of the third boundary dominated flow period. A larger distance from the well to the outer boundary leads to stronger gas production. The real gas has non-obvious effect on gas production. Both the stress dependence and humidity have negative effects on production curves, which are induced by the decrease of the effective pore radius. Specifically, compared with the gas production without considering water film effect, when the humidity value is 0.1, gas production can achieve an average decrease of 18.57%. When the humidity value is 0.3, gas production can reach an average decrease of 26.10%.