Summary Some early high-inclination wells were impacted adversely by wellbore instability within Cretaceous mudstones overlying the Clair oil field. Detailed investigations of the possible reasons for borehole failure led to a decision to cut two cores within the unstable interval and carry out laboratory testing to investigate the failure mechanism. Analysis of rock failure based on a "planes of weakness" theory fitted previous drilling observations. This theory has been applied primarily for layered hard rocks (e.g., mining and tunneling) and is here extended to significantly weaker mudstones showing strength anisotropy. Minimum required mud weight predictions were made for a range of well configurations. The results of this study were used to plan an extended-reach-drilling (ERD) development well targeting a previously undeveloped part of the Clair field. The new well was successfully drilled, cased, and brought on stream within budget in 2009. This paper describes the basic geology of the field, rock properties of the Cretaceous mudstones, details of laboratory tests conducted, and analyses carried out. Cretaceous mudstones of the Clair field are characterized by low strength and minor anisotropy. Various rock tests were conducted on core to define the failure behavior: unconfined compressive strength (UCS), single-stage triaxial strength, and multistage triaxial strength tests were conducted, all at various angles to bedding. In addition, the effect of mud invasion was analyzed by performing a multistage triaxial test in drilling mud at high pressure. The analysis used commercial software to evaluate borehole instability in a rock medium that fails along weak planes. Minimum mud weight requirements derived from this analysis were compared with previous drilling experience and were used subsequently for predicting mud weight required to drill an ERD well. Lessons learned from the successful drilling of this well will be applied in future Clair development wells and may be of relevance elsewhere.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWe present a heuristic method of delineating reservoir regions with favorable production potential for well placement. The method uses basic concepts of proxy formulation for well productivity to relate the production potential of a well placed in any region in the reservoir, to the basic properties of that region (petrophysical, dynamic, geometrical). These properties may be readily obtained from any history-matched or dataconditioned reservoir model. Therefore, a productivity potential map for the reservoir can be easily generated. Also, the relevant properties relate to parameters measurable in an LWD operation. We use this technique to simulate the impact of alternate steering strategies on well path and productivity. This requires a fine-scale model of the region designated for drilling (i.e., model prior to upscaling). The influence of additional LWD measurements on reservoir trajectory and expected well performance can therefore be assessed with this technique (e.g., impact of NMR-derived permeability). We also examine how this method may be applied when multiple geological scenarios are conceivable for the area targeted for drilling. We apply a simple technique for model aggregation that proves effective in identifying drainage areas that are optimum or near-optimum in an overall sense. Therefore, a well placed in the region identified by the aggregate model shows a performance with respect to the underlying geological models that is best or not far from it.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHeuristic technique may be called as an "intelligent guess", which reduces the search for the right answer of a usually complex problem. Such techniques are aimed to dramatically reduce the time required to solve a problem. This paper examined the Productivity Potential Map (PPM) technique as a heuristic approach tool in assisting the reservoir engineer to choose optimum well locations for field development planning. Eleven well locations were selected using PPM technique and compared with the same number of wells located using conventional STOIIP based methodology.The exercise used information from six vertical wells in Field A, Otter Sandstone Formation (Sherwood Group). It is a discontinuous fluvial system with highly variable permeability patterns which poses a significant challenge for selecting well targets.A comparison of simulated flow performance showed that well locations selected using PPM technique exhibied consistently better performance and minimum variance. STOIIP and PPM techniques resulted in two totally different templates for the selection of well locations. PPM technique was coherent with the expected geological trend of the area than the trend showed by the STOIIP technique.The exercise showed that the PPM technique is a valuable method in delineating reservoir regions with favourable production potential for well placement, especially in complex sedimentary environments.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractOne of major field development decision is choosing the number of wells required to efficiently drain an oil or gas reservoir. It is an interactive process in which various development scenarios are chosen and their performance analysed. Formation geology and zone connectivity have a major impact on the choice of well location since they determine well productivity. The industry's current well placement selection process is time consuming and costly. It requires analysing numerous development options by performing a large number of flow simulations.This paper describes a technique to partially automate this well placement process. It has been found that a new map which ranks the reservoir zones based on their productivity potential can speed-up, and hence reduce the cost, of this decision making process. This map, termed the Productivity Potential Map (PPM), is based on fundamental petroleum engineering principles. It is generated from the numerical reservoir models developed from standard data measured during the exploration and appraisal process. It incorporates both static and dynamic properties (e.g. porosity and saturation respectively).A static and stochastic numerical reservoir model are coupled with the PPM and well locations that maximise production are identified. The technique was tested using flow simulation models of two UK reservoirs by generating PPM and identifying the drilling targets that could deliver the maximum, sustained production potential.The first example uses a static reservoir simulation model of a field that had been production history matched for 18 years. Compared to the development plan implemented many years ago, the PPM map reduced the number of development wells by 15% while increasing the cumulative oil production by 2.6% at 2.5 years since production started. The second example employs multiple realisations developed from exploration well data using PETREL TM . Well locations were chosen from a PPM map derived from these multiple realisations. The chosen well locations clearly matched the reservoir geology. Well locations were also chosen from a STOIIP based map. The performance of the STOIIP and PPM based field development were compared -the PPM based well placement consistently performed best.PPM thus reduced the flow simulation effort required, improved the flow forecast and reduced the uncertainty in flow performance. This paper will show that the use of a PPM is a quick and cost-effective technique for analysis of the reservoir production performance and the generation of drilling targets. The technique may also aid reservoir management decisions e.g. water flood front location and selecting the preferred water flood front direction.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractOne of major field development decision is choosing the number of wells required to efficiently drain an oil or gas reservoir. It is an interactive process in which various development scenarios are chosen and their performance analysed. Formation geology and zone connectivity have a major impact on the choice of well location since they determine well productivity. The industry's current well placement selection process is time consuming and costly. It requires analysing numerous development options by performing a large number of flow simulations.This paper describes a technique to partially automate this well placement process. It has been found that a new map which ranks the reservoir zones based on their productivity potential can speed-up, and hence reduce the cost, of this decision making process. This map, termed the Productivity Potential Map (PPM), is based on fundamental petroleum engineering principles. It is generated from the numerical reservoir models developed from standard data measured during the exploration and appraisal process. It incorporates both static and dynamic properties (e.g. porosity and saturation respectively).A static and stochastic numerical reservoir model are coupled with the PPM and well locations that maximise production are identified. The technique was tested using flow simulation models of two UK reservoirs by generating PPM and identifying the drilling targets that could deliver the maximum, sustained production potential.The first example uses a static reservoir simulation model of a field that had been production history matched for 18 years. Compared to the development plan implemented many years ago, the PPM map reduced the number of development wells by 15% while increasing the cumulative oil production by 2.6% at 2.5 years since production started. The second example employs multiple realisations developed from exploration well data using PETREL TM . Well locations were chosen from a PPM map derived from these multiple realisations. The chosen well locations clearly matched the reservoir geology. Well locations were also chosen from a STOIIP based map. The performance of the STOIIP and PPM based field development were compared -the PPM based well placement consistently performed best.PPM thus reduced the flow simulation effort required, improved the flow forecast and reduced the uncertainty in flow performance. This paper will show that the use of a PPM is a quick and cost-effective technique for analysis of the reservoir production performance and the generation of drilling targets. The technique may also aid reservoir management decisions e.g. water flood front location and selecting the preferred water flood front direction.