The ultra-deepwater fields in the Gulf of Mexico are among the largest producers discovered to date. However, the reservoirs are interbedded with highly depleted zones, with pressure differentials up to 11,000 psi. The development, testing, and application of wellbore stability modeling software that accurately characterizes fractures, determines the optimal lost circulation material (LCM) blend, and delivers reliable wellbore strengthening results in the problematic production zones are discussed. Wellbore strengthening literature focuses on three fundamental areas: stress caging, fracture–closure stress, and resistance to fracture propagation. Aspects of these approaches were incorporated into a new modeling solution that was calibrated using historical data from nine offset wells. The modeled fracture width predictions were used to design lost circulation material (LCM) treatments with specific particle size distribution values. Each formulation underwent particle-plugging testing in the laboratory, followed by flow loop testing of the best performers for compatibility with downhole tools. The highly interactive process, which currently continues, resulted in successful field applications in similarly complex wells. The new model allowed drilling personnel to identify the parameters most likely to induce fractures. Equivalent circulating density (ECD) had the most impact, followed by minimum horizontal stress, Young’s modulus, fracture length, and Poisson’s ratio. Using modeling outputs, LCM blends were engineered to plug fracture widths ranging from 1,500 to 2,000 microns, significantly wider than previous estimates. Field results indicated that an "extended" ECD margin could be obtained for severely depleted formations. The optimized LCM treatments were applied on two wells with narrow pore pressure/fracture gradient margins and on one well with a severely depleted reservoir (4,600 psi). All three were drilled with zero losses. On a fourth well, the modeled treatment was applied to the leak-off test at the 16-in casing shoe above the production zone. The operator expected a 0.4 to 0.5 lbm/gal increase at best; the actual increase was more than 1.0 lbm/gal. After this interval was drilled, a 14 in liner was set and cemented with zero losses. Such an increase had not been possible on offsets previously. Based on these successes under similar conditions, the operator is currently implementing the model to design wells with extreme depletion to be drilled during 2020. Decades of deepwater experience have yielded numerous best practices for drilling in narrow margins and depleted zones. However, many wells still cannot be drilled without an assurance of effective wellbore strengthening. By removing the limitations of other wellbore strengthening approaches, the field-proven geomechanics modeling software presented in this paper creates a new standard for lost circulation prevention in depleted sands with 8,000 to 11,000 psi differentials.
Geothermal assets continue to be developed aggressively in several regions around the world, fueled by the need to develop clean sources of energy and the recognition that geothermal energy is one such source. As expected, most Geothermal projects are planned and executed using legacy oil/gas drilling systems, tools, and strategies. While effective in some instances, this approach introduces risks, limitations, and inefficiencies into the geothermal drilling process. Consequently, project cycle times and operational costs are usually higher than expected, with additional negative effects on energy production levels and efficiencies. These challenges need prompt solutions, which must be achieved through development of geothermal specific technologies and strategies. Such considerations are needed to promote drilling efficiency improvements, sustainable project cost reductions, and facilitation of expanded geothermal asset development. In support of required efforts, this paper presents structured processes that guide the interpretation and understanding of geothermal drilling applications and promote holistic development of geothermal specific solutions. The positive effects of a holistic approach and other processes on geothermal drilling, with a focus on improved efficiencies, cycle time and operational costs reductions, will be presented with supporting field data.
As the Managed Pressure Drilling (MPD) systems for deepwater drilling rigs mature, operators are applying the technology on more complex prospects. Wells are encountering higher pressures in deeper water depths, pushing against the boundaries of technical limits not previously encountered. A prospect in the US Gulf of Mexico required drilling to measured depths exceeding 31000 feet in water deepwater. Under such demanding depth, a non-typical drillstring was required to manage the tensile loading. Typical drill pipe connections on 6 5/8" S-135 tool joints are 8.5" diameter. This drill string would require V-150 landing string, with a 6 5/8" FH tool joint diameter of 8.875". Hard banding would bring the tool joint nominal OD above 9". The depth of the well and planned string RPM presented risk of casing wear, therefore drillpipe protectors would also be required. The depth of the reservoir and size of the drillstring meant pipe would need to be stripped out of the well with up to 900 psi backpressure in order to maintain constant bottom hole pressure. All well challenges were used to determine design specifications for a custom sealing element. The scope of work was to design, validate through finite element analysis, then validate in a test fixture per API16RCD test procedures. On conclusion of the product validation, a land test rig trial, with mock-up of the planned system, including dual sealing elements in the Rotating Control Device (RCD), the required non-rotating drill pipe protectors on the planned drillpipe, was executed. The development schedule from start to finish was compressed to less than 6 months also, targeting completion ahead of the rig's drilling program. This paper will recount the various phases of the design-build-validate-test effort that went into resolving these technical limits. It will conclude with field results and lessons learned from first deployment. As operators pursue more challenging deepwater wells, this systematic approach, through alignment of the operator, drilling contractor and MPD technology company, serves as a model to expand the operating envelope of drilling systems, improving safe performance in a cost-effective manner.
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