No abstract

Borehole acoustic waves are affected by near- and far-field stresses within rocks that exhibit stress sensitivity, typically in medium- to high-porosity formations. Nonlinear, or third-order, elastic constants are obtained from the inversion of borehole sonic shear radial profiles with an elastic wellbore stress model. The stress-to-velocity relationship determined from these profiles in the elastic region surrounding the wellbore is used for calibration to compare with empirical laboratory data traditionally used in time-lapse seismic-feasibility studies to assess simulated production. This analysis enables rock physicists to use the wellbore as a laboratory and to examine the stress dependence of the acoustic velocities from in situ field data in their zone of interest. Laboratory experiments on core samples can yield both empirical and mathematical rock-physics models to describe the relationship between stress and velocity to link rock properties to in situ measurements of acoustic data (seismic and sonic). In an example from offshore Malaysia, full-waveform borehole sonic data are processed to produce shear radial profiles in a deepwater environment. The compressional velocities are mainly sensitive to stress in the polarization-propagation direction, and shear velocities are mainly sensitive to stresses in propagation and polarization directions, as expected from nonlinear elasticity. The three compressional and shear velocities vary greatly with vertical stress depending on the stress path because they depend on the three principal stress magnitudes. In contrast, a classical empirical model that depends on porosity, clay content, and effective stress cannot capture differences caused by stress path because it relies on only one stress. Results show that stress sensitivities are significantly stronger with borehole radial profiles than the empirical model for all considered stress paths (K = −0.5, 0, 0.5, and 1).

No abstract

As fields mature, drilling can become more difficult. The likelihood of losses increases as reservoir pressures decline while higher mud weights are needed to prevent collapse of overburden shales as targets are pushed further from the platform. Drilling parameters for the Forties field have become fairly well established after years of experience yet 65% of the wells drilled between 2002 and 2007 experienced incidents attributed to instability. As field production declined, economic viability demanded a step change in performance. Through a better understanding of the field geomechanics and past drilling events, the Apache drilling team has implemented fit for purpose drilling procedures that have significantly improved drilling efficiency. This paper describes how geomechanics analysis has been used to assist well planning. Incorporating the experience of recent wells, the Forties mechanical earth model has been refined and provides key inputs needed to optimise well plans and adapt drilling practices to changing conditions. The adverse effects of anisotropy is a key reason for wellbore instability in the overburden. A joint Apache-Schlumberger team has been working to integrate geomechanics and geophysics knowledge of the field to quantify the effects of anisotropy related to bedding planes and weak shales. It can be difficult to predict drilling and completion risks during well planning, often due to the lack of distinction between events caused by formation instability and those which are drilling induced (drilling practices, hole cleaning). Capturing and classifying historic drilling events is a fundamental key process in understanding the mechanism and causes of well bore failures. Drilling performance in this mature field has been improved through the collaborative effort of Apache staff and key service providers. The refinement of the wellbore stability model has enabled sound practices and procedures to be developed.

Success in developing and maintaining wells in mature fields is heavily dictated by cost reduction to maintain profitability and maximize production life of the filed. Achieving this relies on better well planning and optimized drilling performances with reduced drilling non productive time. Pertinent to this is the ability to understand mechanisms of wellbore failures, predict and manage operational risks such as wellbore instability, sand production, or hydraulic fracturing. This paper describes how analysis of the historic drilling events have been incorporated to refine mechanical earth models and provides key inputs needed to optimise well plans and adapt drilling practices to changing field conditions. Capturing and classifying historic drilling events is a fundamental key process in understanding the mechanism and causes of well bore failures. A workflow has been engineered to perform such task systematically and efficiently in a field scale multi-well environment. The users are provided with a collaborative common environment to visualize and understand relationships between drilling events in an earth model context. Driven by this new-found understanding, a more accurate prediction of where, when, and how specific problems are likely to manifest themselves can be made. Ultimately it enables closer collaboration between domains and the knowledge gained can be rapidly incorporated into future well planning and drilling operations, while mitigating risks and therefore reducing costs. Introduction Extending the life of mature fields is dependant, not only on carefully selecting locations for optimized production, but also on reducing the cost of operations. As the giant Forties field in the UK sector matures, drilling is becoming increasingly difficult and complex. Increased likelihood of losses as reservoir pressures decline calls for drilling with low mud weight. However the mud weight needs to be high enough to prevent the overburden shales from collapsing as targets are pushed further away from the platform. To compound this, the anisotropic behaviour of shales in the Forties area has had an adverse impact on the wellbore stability in the overburden. With an aggressive drilling campaign for 2009, Apache faces a tough task to improve drilling efficiency and practices to succeed in this challenging drilling environment. This has placed a prime importance on well planning and drilling procedures that translates to better execution in the drilling phase and hence reduction in operational costs. This has been achieved through better understanding of geomechanics behaviour and risk analysis, not on an individual well by well basis but in the context of the drilling environment of the entire field. For the case of the Forties field, this in part involved a comprehensive review of the historic drilling events and performance of over 100 wellbores drilled since the acquisition of the Forties field by Apache in 2003. Clearly a systematic and methodical approach is needed for accurate evaluation of wellbore failures not only because of the number of wellbores involved, but also because wellbore failures can be attributed by a whole host of factors or combination of them. These factors can be broadly defined as field geological factors or drilling operation factors. Without proper diagnosis it would be difficult to confidently predict and mitigate these risks in the well planning phase and ultimately will have an adverse impact on drilling operations.