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.
Summary A post-mortem analysis of the Gnu-1 well was conducted to help us to understand drilling experiences in the context of the pore-pressure and stress profiles. The post-mortem involved a review of the drilling experiences and an analysis of CAST image data, wireline-log data, and the logging-while-drilling (LWD) logs. This information was used to refine and verify a geomechanical model (in-situ stress, pore pressure, and rock-mechanical properties) in the vicinity of the Gnu-1 well. Of prime concern was the verification of the predrill pore-pressure prediction previously undertaken using 3D-seismic-velocity data and offset-well data. Wellbore-failure and natural-fracture analyses were integral parts of the post-mortem. Wellbore breakouts seen in the image data allowed the pore pressure in the 8.5-in. hole section of Well Gnu-1 to be constrained. Modeling using image data collected in the Athol formation indicates that the pore pressure does not increase as rapidly as was estimated in the predrill study. Pore pressures in the North Rankin formation and below were consistent with the predrill study. The geomechanical model was able to explain the losses seen in the Athol formation in Well Gnu-1 when using the mud weights experienced by the open hole at the time of drilling. Introduction The Gnu prospect is situated in the northern portion of Block WA-209-P in the Dampier subbasin, Australian northwest shelf (Fig. 1). The prospect is located within the Reindeer gas field. A number of offset wells exist in the region, the closest wells being Well Reindeer-1 (approximately 1.5 km to the northeast) and Well Caribou-1 (2 km to the southeast). Well Gnu-1 was designed as an exploration well. The anticipated overburden stratigraphy at the location of Well Gnu-1 consists of Tertiary and Upper Cretaceous carbonates, marls and siltstones that overlie Cretaceous claystones, siltstones and minor sandstones, and greensands. The primary aim was to drill vertically to intersect the Muderongia australis glauconitic sandstone and then to build angle and continue drilling a deviated hole through the main Reindeer field gas appraisal within the Legendre formation and into the North Rankin, Brigadier, and Mungaroo formations.
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.
A postmortem analysis of the Gnu-1 well was conducted to help understand the drilling experiences in the context of the pore pressure and stress profiles. The postmortem involved a review of the drilling experiences, the analysis of CAST image data, wireline log data and the LWD logs. This information was used to refine and verify a geomechanical model (in-situ stress, pore pressure and rock mechanical properties) in the vicinity of the Gnu-1 well. Of prime concern was the verification of the pre-drill pore pressure prediction previously undertaken using 3D seismic velocity data and offset well data. Wellbore failure and natural fracture analysis were an integral part of the postmortem. Wellbore breakouts seen in the image data allowed the pore pressure in the 8½" hole section of Gnu-1 to be constrained. Modelling using image data collected in the Athol Formation indicates that the pore pressure does not increase as rapidly as was estimated in the pre-drill study. Pore pressures in the North Rankin Formation and below were consistent with the pre-drill study. The geomechanical model was able to explain the losses seen in the Athol Formation in Gnu-1 when using the mud weights experienced by the open hole at the time of drilling. Introduction The Gnu Prospect is situated in the northern portion of Block WA-209-P in the Dampier Sub-basin, Australian Northwest Shelf (Fig. 1). The prospect is located within the Reindeer Gas Field. A number of offset wells exist in the region, the closest wells being Reindeer-1, approximately 1.5 kilometres to the northeast, and Caribou-1, 2 kilometres to the southeast. Gnu-1 was designed as an exploration well. The anticipated overburden stratigraphy at the location of Gnu-1 consists of Tertiary and Upper Cretaceous carbonates, marls and siltstones that overlie Cretaceous claystones, siltstones and minor sandstones and greensands. The primary aim was to drill vertically to intersect the M.australis glauconitic sandstone and then to build angle and continue drilling a deviated hole through the main Reindeer Field gas appraisal location within the Legendre Formation and into the North Rankin, Brigadier and Mungaroo Formations (Ref. 1). The M.australis sandstones lie within a stratigraphic trap, formed due to restricted deposition and subsequent erosional truncation in the vicinity of the Reindeer Gas Field. The traps at other levels are structural closures. The Legendre Formation comprises blocky well developed sandstones which are anticipated to have similar porosities and permeabilities to those rocks seen within the Reindeer-1 and Caribou-1 and Caribou-1ST-1 wells. The purpose of drilling to specific depths within the North Rankin and Brigadier Formations was to test the potential of an additional structural closure. Both the North Rankin and Brigadier Formations were expected to contain hydrocarbons and also different degrees of overpressure. In fact, the main drilling hazard for the deeper targets was the overpressure which was expected to be encountered based on information obtained during the drilling of Caribou-1. The nearest well, Reindeer-1, did not drill deep enough to intersect these overpressured formations. The objectives of this paper are to show how geological and well engineering data from the exploration and appraisal wells in the Dampier Sub-basin were used to build a pre-drill geomechanical model for the Gnu Prospect to help in the design of the Gnu-1 well. The postmortem analysis shows the strengths and limitations of the pre-drill pore pressure from high quality 3D seismic velocity data. The analysis was subsequently verified and improved using observations of wellbore breakouts in image data to back calculate pore pressure in the shale. Geomechanical Approach A robust, field-specific geomechanical model (in-situ stress, pore pressure and rock mechanical properties) has proved to be an essential tool for the hydrocarbon industry (Ref. 2). This is because geomechanics has an impact on all aspects of field development from the initial exploration and appraisal stages to production and field abandonment. A properly optimised development plan incorporating geomechanics can result in large cost savings over a field's lifetime.
fax 01-972-952-9435. AbstractA postmortem analysis of the Gnu-1 well was conducted to help understand the drilling experiences in the context of the pore pressure and stress profiles. The postmortem involved a review of the drilling experiences, the analysis of CAST image data, wireline log data and the LWD logs. This information was used to refine and verify a geomechanical model (in-situ stress, pore pressure and rock mechanical properties) in the vicinity of the Gnu-1 well. Of prime concern was the verification of the pre-drill pore pressure prediction previously undertaken using 3D seismic velocity data and offset well data. Wellbore failure and natural fracture analysis were an integral part of the postmortem. Wellbore breakouts seen in the image data allowed the pore pressure in the 8½" hole section of Gnu-1 to be constrained. Modelling using image data collected in the Athol Formation indicates that the pore pressure does not increase as rapidly as was estimated in the pre-drill study. Pore pressures in the North Rankin Formation and below were consistent with the pre-drill study. The geomechanical model was able to explain the losses seen in the Athol Formation in Gnu-1 when using the mud weights experienced by the open hole at the time of drilling.
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