Abstract
Reservoir depletion will cause the safe equivalent circulation density (ECD) operating window of drilling fluids to narrow, or even disappear. Previous studies have proposed a set of two specific casings at the top and bottom of the depleted reservoir, respectively, or conducted wellbore strengthening to increase fracture pressure, but these will cause a waste of time and costs, or differential pressure sticking. Aiming at resolving this problem, a novel concept and evaluation method of moderate collapse in the shale cap was developed and case calculations were performed. The results show that the degree of collapse is different for wells drilled in different types of fault regimes, and it can be controlled by optimizing the well trajectory. The collapse pressure within the shale cap was decreased due to reservoir depletion, and when a certain degree of collapse was acceptable, the collapse pressure can be even lower and a safe operating window will appear which can be beneficial to optimizing the casing program and drilling design. The research results provide a theoretical basis and new design idea for successfully and economically drilling into new untapped reservoirs in deeper horizons through depleted zones in the future.
Highlights
At present, more than 70% hydrocarbon resources are being obtained from secondary and tertiary recovery [1]
The results indicate that for all the wells drilled in different types of fault regimes, the collapse pressure gradient will be gradually decreased with the reservoir depletion, and when 50% reservoir pressure is depleted, the collapse pressure gradient will be decreased by about 20%
Production from hydrocarbon reservoirs can lead to a decrease in pore pressure unless pressure support is provided from a gas cap or an aquifer
Summary
More than 70% hydrocarbon resources are being obtained from secondary and tertiary recovery [1]. In order to avoid the specific casing set and make it allowable that the shale cap and depleted reservoir be in the same open hole, new ideas, such as wellbore strengthening, have come into being which can enhance the fracture gradient in the depleted reservoir to extend the drilling margin [6,17,18,19,20,21,22]. The approach can reduce the risk of lost circulation, the required higher ECD to maintain the mechanical stability in the overlaid shale cap may trigger differential pressure sticking in the depleted reservoir due to its lower pore pressure. By this time, it is hoped that the ECD to be as low as possible. The moderate collapse concept and design method in the shale cap near the depleted reservoir were proposed in this paper so as to extend the allowable drilling margin and provide a reference for drilling design in depleted reservoirs
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