Using Thixotropic Cement Slurry for Lost Circulation Control: Case History, Volga-Urals Region

Abstract

Drilling operations within the Volga-Urals oil and gas province have a history of drilling problems manifested in partial and total losses of drilling fluid. As a result of these challenges, the companies developing and operating oil and gas fields within the region encounter the following: Significant nonproductive time (NPT) losses and additional financial costs (Urdaneta et al. 2015)Low-quality casing cementing operations proven by sonic cement-bond logging (CBL)Costly remedial cementing and sidetracking or redrilling operations because drilling ahead is no longer possibleWell abandonment, etc. Large-size particle-bridging materials are not efficient for blocking thief zones with wide fracture openings or vast cavernous intervals because they exceed the size of bridging material (Canson 1985). Technologies based on a different principle are necessary to enable efficient plugging within fractures of such thief zones (e.g., special-purpose cement-slurry-based fluid systems with distinctive thixotropic properties developing high gel strength in a short time to assist in controlling and to help reduce drilling fluid losses of any magnitude). During 2016, in cooperation with the operating company, a decision was made to conduct pilot field trials of a new method. The new special-purpose thixotropic cement slurry used within the Volga-Urals oil and gas province is a fluid characterized by low content of solid abrasive particles and by unique rheology. This slurry becomes fluid as soon as shear is applied to it and remains fluid while in dynamic state, such as when pumping downhole or circulating in an averaging tank. While shearing force is reduced, slurry viscosity increases. This fluid forms an internal gel structure after a short static period followed by intensive gelling and gel strength (shear force) development. The slurry can be squeezed into the thief zone through the bottomhole assembly (BHA), saving tripping time (Urdaneta 2016). Plug slurry density can be adjusted from 1.2 to 1.8 g/cm3 for service temperature within the 38 to 110°C range, perfectly matching virtually all downhole conditions within the region (Duffy et al. 2017). This thixotropic cement slurry formula has a simple composition and dissolves well in water using a dedicated averaging tank provided with a circulating mixing system. Owing to distinctive thixotropic properties of the slurry, its performance at downhole conditions (temperature and pressure) is verified using laboratory high-pressure/high-temperature (HP/HT) consistometer testing (on-off-on mode). The on-off-on test helps clearly define how thixotropic properties of the lightweight thixotropic slurry manifest during the operation. Signature features of this test are distinct spikes in slurry consistency (Bc) recorded on the thickening diagram after a short static period. At the same time, this thixotropic cement slurry is easily reversed to a fluid state by resuming circulation or by applying some shearing force to it. After waiting on cement to harden, the cement stone possesses relatively high strength, reaching up to 300 psi, which helps resume drilling ahead as soon as fluid losses are reduced. The new technology has been used for multiple fields of the operating company's with positive outcomes (i.e., fluid losses mitigated completely or partially). The cement slurry demonstrated rapid gel strength development in downhole conditions, with fluid losses reduced for one or two operations. As a result, the operator decided to proceed with the rollout of this method. The primary limiting factor for its use is the 15.9-m3 volume of the averaging tank necessary to mix the thixotropic cement slurry. Advantages of the thixotropic cement slurry used for lost circulation control include: Mixes easilyCan be pumped through BHA with telemetry tools and drill bitFeatures high gel strength and relatively high compressive strength when setLow content of abrasive particles These features help efficiently reduce drilling NPT associated with lost circulation problems. The first pilot operations completed within the Volga-Urals oil and gas province of Russia demonstrated high process performance and cost efficiency.