Re-Activation of Fractured Gas Reservoir with Active Bottom Water at the Late-Stage Development: A Success Story from Weiyuan Gas Field, China

Abstract

Abstract It is widely recognized that economical gas production from water-drive gas reservoirs is a challenging task due to water incursion, especially for naturally fractured gas reservoirs with active bottom water. The presence of unwanted water production leads to early abandonment of these reservoirs, and their recovery factors are typically below 50%. Enhancing gas recovery has been a constant topic for petroleum engineers. In this work, a new development strategy has been presented to reactivate water-invaded gas reservoirs using a combination of horizontal and vertical wells. Around the water-breakthrough regions, horizontal wells are drilled with the target point at the original gas-water transition zone to produce the invaded water. The surrounding old vertical wells are then rescued to produce gas again. Microscale porous simulation and macroscale reservoir simulation have been conducted to reveal the re-activation mechanisms, and the successful application case in Weiyuan gas field has been analyzed in detail. Using the VOF (Volume of Fluid) calculation method and digital rocks, the invasion pathways of the bottom water up to the gas reservoir have been tracked, and quick water intrusion through the fractures has been observed. The invaded water is easily produced after drilling a horizontal well due to its large drainage area. As a result, the gas production rate of the original vertical wells (typically sited at the top of the reservoir) experiences a rise. Microscale two-phase flow behaviors are consistent with the reservoir simulation results of Weiyuan gas field, where the water saturation of the entire reservoir is significantly decreased if a horizontal well is implemented to produce water. In the gas field, 8 horizontal wells were drilled along the water-breakthrough regions during 2008-2014. The good field response indicates the strategy’s success since the original vertical wells in the near-horizontal-well region have returned to producing gas, confirming that the invaded water is produced and further water intrusion is avoided, preventing damage to the upper gas reservoirs. This proposed method offers a solution to the nearly abandoned carbonate gas reservoir, providing the possibility of further recovering the remaining gas resource. The successful application in Weiyuan gas field can serve as a valuable reference for similar types of gas reservoirs worldwide.