Curing Lost Circulation with an Engineered Spacer During Cement Placement

Abstract

One of the arts of providing zonal isolation in wells is the ability of the cement to fully replace the mud system in the annulus. One way of achieving this is to pump a spacer system that can displace the mud, which means the spacer system should be able to generate enough downhole force to overcome the yield stress of the mud. To achieve effective zonal isolation, the cement spacer must remove both movable and immovable mud (that may have gelled up during static conditions) from the wellbore before cementing. To achieve this, the mud must be conditioned through circulation so that the rheological properties of the mud in and out of the well are uniform. Challenges are usually encountered in wells where losses are experienced during drilling or while running casing. Usually, attempts are made to cure such losses before the cement job. Unfortunately, some losses cannot be cured with lost-circulation materials before the cementing operations. When such situations arise, it becomes difficult to condition the mud or circulate the mud to alter the rheological properties, as circulation can result in further break down of the formation, thereby leading to more severe losses. A similar challenge occurred during operations for an operator in Kenya. After drilling the well to the expected total depth, during the casing run at a flow rate of 150 gpm, total losses were observed. To solve this problem, an engineered cement spacer that can cure moderate to total losses was deployed during the cementing operation. This unique spacer system can form a seal across the sand face and can be designed with rheological properties optimized to enhance the displacement of unconditioned mud. This paper will describe in detail the unique nature of this cement spacer in curing losses as well as the mechanism with which it provides its sealing effect in permeable and fractured formations. Case studies will be presented to describe how the spacer has been used to cure moderate to total losses in oil, gas, and geothermal wells in formations with low fracture gradients and induced fractures.