Drilling Depleted Reservoirs - Is Formation Damage Sometimes a Good Thing?

Abstract

Drilling depleted reservoirs can present many challenges from a drilling, completion and production perspective. Formation damage due to increased overbalance for example, is well documented as a particular concern. But could we see some drilling benefit from increased formation damage at higher overbalance? This paper will outline experience in drilling depleted reservoirs and discuss the impact of formation damage on well productivity, injectivity and drilling performance. In several examples, the benefits of formation damage outweigh the deficits and we propose a planned approach for future wells in depleted reservoirs. Formation damage is measured in the same way as for assessment of the productivity or injectivity impact but the data is applied to a different challenge. The additional pressure drop in the near wellbore area as a result of formation damage can protect the reservoir and minimise fracture propagation. Whilst formation damage can be important in depleted reservoirs it is largely irrelevant if wells cannot be drilled. With the method proposed wells can be planned, drilled, completed and produced (or injected).The paper proposes better understanding of the impact of formation damage in order to enable drilling, completion and development of depleted reservoirs or intervals and proposes a new methodology to use formation damage as part of a well planning workflow. IntroductionIn terms of delivering a functional, commercial wellbore the drilling process and the fluid systems employed therein has the single biggest effect on well quality and in turn well productivity. As a function of that we strive where possible to mitigate the influence of drilling induced well bore formation damage. However, there is a paradox here in that there is a case for planning and deliberately inducing formation damage as part of a well planning workflow, consequently strengthening the depleted section of the openhole as a function of improved hoop stress strengthening the wellbore to facilitate otherwise un-drillable wells.The objective of this deliberate act is to strengthen depleted reservoir or naturally weak formations (e.g., chalk) that must be drilled through to access reservoir(s) of interest as part of the 'transport length' in typically infill drilled wells to allow wellbore to access deeper and higher pressure horizons. In cases like this, the mud weight required to maintain borehole stability across exposed shale sections or to drill through objective sand will create excessive ECD across depleted sand higher up in the open hole. Here, wellbore strengthening will help increase fracture pressure in the depleted sand and sustain higher mud weight.Mud formulation for wellbore strengthening in the reservoir to be produced will necessarily differ from what can be used drilling through depleted sand or fractured Carbonates in the transport length section of a well. In these non-reservoir intervals, it is often "desirable" to drill with mud systems that promote formation damage. Some mud systems, such as SILICATES, a...