An Integrated Approach to Tackling the Challenges of Drilling in a Highly Depleted Gas Reservoir

Abstract

In many matured fields around the world, the infill well development faces multiple challenges; reservoir depletion caused by existing wells is unavoidable, the remaining area to be targeted by infill is typically relatively marginal, with thinner formations that still bear significant subsurface uncertainty compared with sweet spot area developed in the initial phase. In addition, some fields are impacted by PSC conditions, with a strict time constraint. When developing infill wells several key aspects have to be considered: starting from potential severe mud losses, drill pipe stuck during the drilling phase, to formation damage, production interference with neighboring wells, earlier load up for gas wells etc. Shell China developed an integrated approach by considering all these challenges, and successfully implemented it for the Changbei tight gas infill well project. During the Design Phase, a series of core lab tests were carried out to evaluate the formation damage and related permeability reduction. The lab test results indicated that the permeability reduction as result of water encroaching into water wet rock formation is significant (+90%), causing a "water locking" effect. Another topic pertaining to the Desing phase in the Changbei field is the optimisation of the dual lateral well trajectory based on the expected depletion state of the reservoir. Also discussed is the horizontal well tubing size optimization, which accounted for the selection matrix based on KH, online date (related to PSC end) and expected reservoir pressure (depleted). In the Delivery phase, the surfactant additive identified through the lab testing has been used into the drilling mud and completion fluid to appropriately mitigate the water locking effect. The lab test results demonstrated that a permeability improvement of at least 10% could be achieved. Furthermore, the surfactant concentration was optimized to maximise the emulsion effect for water treatment and the foaming effect during the drilling phase. This paper also covers the well flowback efficiency improvement achieved by additional nitrogen lifting and prolonged firing time. During the Well Reservoir and Facility Management phase (WRFM), a study of infill well production interference with existing wells was carried out and the recovery could be maximized at the cluster level.