The Hidden Threat of Irreversible Water Blockage in Low Porosity Gas Fields: Learnings from Pre-Production Paleo-Imbibition for Post-Production Field Management in Oman

The high residual gas trapping characteristics of low porosity rock are a considerable challenge for recovery of fields with bottom or edge water. Water lifting and handling is often a practical issue. Perforated intervals may suffer from permanent loss of gas mobility related to hysteresis effects due to imbibition of water (either production induced or paleo-residuals). The discrimination of "trapped" versus mobile gas intervals in a low porosity setting is not straight forward because data can be very deceiving: water imbibed zones may look alike free gas zones, but performance and recovery are quite different This paper presents a show case of a clastic low porosity formation buried at 4Km depth. Following disappointing performance of the wells due to unexpected water production, the interpretation of pre-production fluid-fill-cycle revealed that a large percentage of the GIIP had been trapped over geological time due to tilting of the structure. Ignoring paleo-imbibition resulted in over-prediction of the recovery and misplacement of wells. Further imbibition of water into prolific zones during production of the field may result in early irreversible water loading of the wells with initial high rates. A cross-discipline workflow has been put together to quantify field potential and water risk. A fit for purpose reservoir simulation with a focus on the fluid-fill-cycle can explain the performance of the wells to date and predict the expected recovery from paleo-residual volumes. Down flank co-production of gas and water has been proposed as mitigation for water blockage issues: 1st) to potentially delay further water cusping through prolific zones and 2nd) to enhance remobilization of gas trapped in the paleo-residuals zone. The reported example illustrates the risks of water for gas recovery in low porosity formations, as learnt from a field with paleo-imbibition. We describe diagnostic tools for timely discrimination of paleo-trapped gas volumes to be considered prior to investment decisions, well placement and completions; we suggest fit for purpose models that can be constructed to represent rock and fluid physics without embarking into comprehensive full-field full-hysteresis models; we draw attention to specific permeability modeling guidelines for reservoirs containing significant volumes of low porosity rock to avoid overestimation of production plateau time. We recommend the planning of co-production of gas and water in flank wells to safeguard production from prolific zones in low porosity reservoirs with an aquifer.