Economic production from liquid-rich shale reservoirs is viable only through sustainable oil and gas rates over the reservoir life. One of the critical challenges to economic production from these reservoirs is condensate dropout below the fluid dew point pressure (Pdew), which leads to condensate blocking. This not only degrades well productivity, but also leads to significant liquid losses because of the high critical condensate saturations experienced by tight formations. This article focusses on the impacts of condensate dropout in tight shale formations with fracture networks and identifies potential mitigations based on both compositional PVT process simulations and sector-model reservoir simulations.
Condensate blocking may be mitigated through solvent injection (e.g., propane, CO2, or dry/enriched gases). Injected solvent can suppress Pdew, delaying liquid dropout or enhancing ‘dropped-out’ liquid recovery through partial vaporization and potential miscibility. We demonstrate these effects through compositional PVT modeling of a rich-gas-condensate and simulating various recovery mechanisms by solvent injection in a tank-model approach.
We conducted detailed reservoir simulation studies on a sector model to confirm the somewhat qualitative results of gas injection obtained through PVT simulation. The reservoir model uses a three-horizontal-well pad and a network of propped and unpropped hydraulic and natural fractures. The simulation input consists of fluid models employed in PVT simulation, fixed routine core analysis, and special core analysis (SCAL) data so that the impacts of injecting various gases and injection strategies can be studied.
Above Pdew, C3 injection significantly lowers the original dew point pressure, delaying or sometimes avoiding liquid-dropout regions altogether during the reservoir life. At lower temperatures (<200° F), C3-mixed fluid may behave as an oil with a significantly lowered bubble point, enhancing liquid recovery. Below Pdew, C3 mixed with the leaner gas in equilibrium with dropped out liquid can develop first or multiple-contact miscibility with dropped-out liquids.
CO2 or enriched gas injection yields similar results. When injected above Pdew, CO2 alters liquid dropout to a lesser extent than propane. However, below Pdew, injected CO2 may potentially lead to multi-contact miscibility to enhance liquid production. In case of enriched gas, single-phase region extends only over a smaller pressure range with limited enhancement to liquid recovery.
Lean or dry gas injection has the potential to vaporize large amounts of dropped out liquids, particularly if the C7+ fraction composition is skewed towards lower carbon numbers, thereby increasing liquid recovery.
Overall, this study indicates that proper solvent injection will mitigate condensate-blocking effects and lead to improved liquid recovery with minimal productivity loss.