No abstract
Velocities derived from the seismic data provide indirect estimation of the formation pressure prior to drilling. The uncertainties in velocity estimation increase with the geological complexity and depth which in turn amplify the margin of error in the pore pressure prediction. Such uncertainties can be reduced by adopting suitable velocity to carry out predrill pore pressure prediction. Several advanced techniques for velocity analysis have been used in this study with varying degree of confidence for pore pressure estimation in a deep water HTHP well.The well was designed to drill to a depth of 5000 m with an overpressured Cretaceous clastic sediment column of 2000 m before reaching the reservoir (water depth 600m, maximum prognosed pressure ~11,000 psi and temperature ~190ºC). In deepwater Krishna Godavari basin, the conventional seismic velocity (Handpicked and stacking velocities) based pore pressure prediction resulted in considerable uncertainties in the older Cretaceous sediments as seen in the earlier drilled wells. This called for an advanced velocity analysis (AVO based and Inversion velocities) to reduce the margin of uncertainties for this study well. Such analysis added values to our understanding of the impedance contrast, temporal and spatial variations of velocity in terms of reservoir and non reservoir inter-relationship. A definitive predrill pore pressure curve, taking into account these geological and geophysical factors was the best estimate for well planning. HTHP well drilling challenges can be constrained by depth of top of overpressure, narrow pore-frac window and large ESD-ECD variations due to high temperature gradient. The definitive pore pressure curve catered to limiting all the three critical parameters as comparison with the post drill pore pressure analysis showed a variation of ±0.5ppg. Future deepwater HTHP prospects can be planned by the similar work flow as the drilling experience of this prospect was satisfactory. 2 SPE 153764
In today’s challenging market conditions, the probability of successful well delivery can be increased and influenced by implementing fit-for-purpose pre-drill and real-time geomechanical solutions. These tailored geomechanical solutions add value to the project by delivering a cost-effective well, with reduced non-productive time (NPT), and a lower risk of health, safety, and environment (HSE) concerns. Geomechanics guided decision making, both in the pre-drill and in the real time phases, has a wide range of applications depending on the complexity present in the drilling environment, e.g., high-pressure, high-temperature (HPHT) regimes, reactive clays, depleted reservoirs, weak shales, highly stressed areas, etc. This paper discusses the application of advanced geomechanics in three specific drilling environments (a) drilling a highly deviated well in a transitional fault regime, onshore the Nile Delta, (b) mitigating wellbore instability caused by reactive shales, in the Middle East and (c) drilling lateral wells in a highly-stressed carbonate formation. The paper also discusses how integrated pre-drill and real-time geomechanical solutions helped in achieving drilling success without adding major cost to the project. In study (a) the operator had successfully drilled many vertical wells in the onshore field on the Nile Delta without significant problems, yet was having severe issues drilling deviated wells. A detailed pre-drill model revealed the possibility of a transitional faulting regime, in association with anisotropic rocks, drilled by a slick Bottom Hole Assembly (BHA), could be a major reason for this. Real-time geomechanics were deployed to validate the pre-drill understanding, along with mud additive recommendations and a slight modification to the drill string. In a different study (b) performed in another onshore Middle East field, there was a challenge to drill high-angle wells through troublesome shale formations, which resulted in various sidetracks and a significant amount of wellbore instability issues. These issues limited well configuration options for field development to near vertical wells. A pre-drill geomechanical study was carried out to understand the root cause of the failures that resulted in customized mud weight and mud type solutions for drilling higher angle wells. With these customized recommendations and later on a 3D Geomechanical model, horizontal wells have been drilled successfully for optimal draining of the reservoir resulting in breakthrough in field development plan. In study (c) there was significant wellbore instability challenges while drilling lateral wells through highly-stressed carbonate reservoir. A comprehensive study helped in understanding the geomechanical behavior. In example highlighted the drilling team was using lower than required mud weights in a horizontal well. The geomechanical model was adjusted considering time and space for specific case using the geomechanical understanding. The focused geomechanical modeling helped to adjust the mud weight. Suitable mud weight along with pseudo real-time monitoring helped in successful delivery of the horizontal well. The three studies presented are onshore. Conventional wisdom for onshore drilling has a bias for low-cost solutions. However, the complexity of each drilling campaign was different. In all the cases the adoption of integrated geomechanics through the planning and operation phase ensured successful project completion with minimal non-productive time (NPT).
Drilling challenges in offshore Nile Delta have been largely documented in the literature. Operators are often confronted with drilling problems related to shale swelling, cavings, tight holes in combination with increased risks of lost circulation in some of the highly depleted formations. The Kafr El Sheikh shale in particular, has been linked to many instances of wellbore instability, due to its mineralogical composition (estimated to be mostly smectite, >70%). From offset well drilling experience, it could also be noticed that insufficient mud weight was often used to drill through the Kafr El Sheikh Shale, causing wellbore failure in shear due to lack of support of the wellbore wall. In the past, multiple mud weight designs have been implemented relying solely on pore pressure as lower bound of the mud window. With the increased use of geomechanics, it has been demonstrated that the lower bound should be taken as the maximum of the pore pressure and borehole collapse pressure, thus accounting for the effects of formation pressure, horizontal and vertical stresses, rock properties as well as wellbore trajectory. It has been proven that slight overpressure is often encountered halfway through the Kafr El Sheikh formation, which would typically result in slightly higher borehole collapse pressures. In the study fields, the operator expressed interest in drilling highly deviated wells (> 60-70 degrees). This raised concerns for increased drilling challenges, especially in the Kafr El Sheikh. A comprehensive and systematic risk assessment, design of a fit-for-purpose solution and its implementation during drilling took place in the fields of interest. Offset well data analytics from the subject fields supported a holistic evaluation of drilling risks associated with the Kafr El Sheikh, providing good understanding of stress sensitivity on deviation, azimuth and lithology. Upon building a robust geomechanical model, calibrated against offset well drilling experience, pre-drill mud weight and drilling practices recommendations were provided to optimize the drilling program. Near real-time geomechanical monitoring was implemented which helped to manage the model uncertainties. The implementation of a holistic risk assessment, including geomechanical recommendations and near real-time geomechanical monitoring, was effective to lead the drilling campaign successfully. As a result, three high angle wells (> 60-70 degrees) were drilled through the challenging Kafr El Sheikh formation without any hole instability. An integrated risk assessment of hole instability, managed in stages (pre-drill and during drilling), has helped to understand and simulate the behaviors of the formation. Proactive decisions have established a controlled drilling environment for successful operations.
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