Remote Real-Time Monitoring of Particle Size Distribution in Drilling Fluids During Drilling of a Depleted HTHP Reservoir

Abstract

Abstract This paper describes the successful use of Focused Beam Reflectance Measurement (FBRM) for remote real-time monitoring and management of drilling fluid Particle Size Distribution (PSD) while drilling in a depleted HTHP gas reservoir in the Norwegian sector of the North Sea. Drilling into depleted reservoirs is challenging as the fracture strength of the reservoir is reduced due to pressure depletion from production. Experience has shown that adding suitably-sized blends of calcium carbonate and synthetic graphite to the drilling fluid can significantly improve the fracture strength. Optimum particle blends and size distributions are designed based on expected fracture and formation pore size. In order to maintain the formation strengthening effect, the PSD of the designed blend must be maintained during drilling and circulation of fluid. Furthermore, the method employed must be able to capture the changes in size distribution from the accumulation of drill solids and the mechanical attrition of particles in circulation. Monitoring and managing PSD effectively while drilling has been hindered by lack of suitable equipment for real-time measurement. The successful application of the FBRM technique has shown that reliable data can be obtained and that these data compare well with other conventional PSD measurement techniques. This paper describes how the online PSD measurements were utilized to improve engineering of the drilling fluid to obtain full effect of the formation strength enhancement provided by the added particulate material. The improved PSD control can be used both to ensure sufficient concentration of correctly sized particles to effectively bridge the pore throats in the sandstone formation and at the same time include an ideal concentration of larger sized particles to plug fractures. Good solids control management allowed efficient screening of particles and when combined with addition of new material, provided full control of the total particle concentration. Introduction As oil-producing fields in the North Sea are gradually becoming more mature, requests for more advanced drilling techniques and equipment to mitigate problems associated with the depleted reservoirs are becoming more commonplace. For example, the current development campaign for the mature parts of the Tampen area in the Norwegian Sea, such as the Kvitebjørn field, is planned drilled using Managed Pressure Drilling (MPD) techniques to reduce overbalance while drilling. Planning of the operation also includes a raised focus on preventative treatments for induced and natural fractures. The control of particle size distribution (PSD) while drilling has become an important issue with respect to particle additions, maintenance and solids control management (Omland et al. 2007). Regular monitoring of the drilling fluid PSD is most convenient if it can be handled on site, which requires a granulometer to monitor at least the trend in the PSD. Electrical or optical methods like laser light scattering can be used; an even simpler technique is wet sieve analysis, which has the advantage that fine drill solids and weighting material can be removed so that the measurements reflect the PSD of the added wellbore strengthening or formation bridging material. Common to all these methods are that they require sampling of the drilling fluid with the inherent problems this entails with sampling accuracy and sample preparation. There is a growing demand within the oil industry to identify equipment that can perform real time measurement of PSD directly in the drilling fluid thus providing the potential for improved monitoring and control of particle additions.