Stratigraphic Control and Formation Evaluation of Horizontal Wells With MWD
Authors: Hammons Lane R.B. , Sellers Deborah Hall , Fisher E. Keith 
Published: December 2007

Summary.
Sandstone reservoirs in the Gulf of Mexico typically do not form horizontal planes, and a predetermined course for a horizontal well cannot ensure that the well path will remain within the producing interval throughout the horizontal section. Detecting approaching bed boundaries and fluid contacts early enough to make the changes necessary to keep the well path in the producing interval is an important technical issue during the planning and drilling of horizontal wells. The first two horizontal wells drilled in the Gulf of Mexico achieved this objective with measurement-while-drilling (MWD) resistivity measurements and with supporting computer modeling for bed boundary detection.

Introduction
MWD propagation resistivity measurements can effectively detect approaching bed boundaries several feet from the borehole if there is a significant resistivity contrast between the two beds. In the well planning stage, the MWD log response can be modeled from the horizontal-well plan and data from offset wells. During drilling, the modeled log response can be compared with realtime directional and log data to monitor and correct the path as necessary. Finally, the MWD log and directional data can be used in postdrilling evaluation (1) to perform a petrophysical evaluation of the well, (2) to determine actual reservoir geometry, and (3) to plan completion and production strategies. Case histories illustrate how a combination of MWD measurements can be used to plan horizontal-well paths, maintain realtime stratigraphic control of the horizontal-well path, and evaluate the reservoir without requiring deployment of openhole wireline logs in the horizontal section.

Background
Texaco Eastern E and P Region has produced gas and condensate from deep reservoirs in the East Cameron (EC) Block 265 field, which encompasses EC Blocks 265, 278, and 283 in the Gulf of Mexico, 195 nautical miles southwest of New Orleans (Fig. 1). Three prolific gas sands were recognized in the field area northwest of Platforms A and B; however, platform placement relative to the shallow subsurface geologic structures made conventional development drilling and production in these three sands difficult. Recent technological advances in horizontal-drilling techniques made it possible to consider extended-reach, horizontal, or near horizontal wells as viable economic alternatives for producing these reservoirs; however, none of these methods had been attempted in the unconsolidated sediments common to the Gulf of Mexico. EC Block 265 field production history indicated probable water coning in one of these large gas reservoirs. A horizontal well was designed for this reservoir to reduce water coning by extending the drainage radius and reducing pressure drop at the wellbore/sand interface. After the horizontal-well plan was developed and the associated engineering issues addressed, two problems remained: how to predict accurately when the drill bit crossed the boundary between the shale bed and the objective gas sand below, and how to determine the location of the wellbore within the objective hydrocarbon section as drilling progressed. A full-service MWD tool was chosen to supply directional, gamma ray, and resistivity data for use in well-path monitoring, correlation with offset wells, and formation evaluation. The use of MWD technology provided a critical advantage: MWD modeling capabilities made it possible to predict tool response, and these predictions could be used as a guide during drilling. With data from offset wells, resistivity values in the objective reservoir were modeled for a 2-MHz MWD dual propagation resistivity (DPR) response at 87, 90, and 93 degrees angles. During drilling, these models were used successfully m combination with real-time data for well-path monitoring, offset well correlation, and stratigraphic prediction.
Why Drill a Horizontal Well?
A typical Gulf of Mexico field development and production plan begins with producing hydrocarbons from the deepest known stratigraphic reservoir and proceeding upward until the shallowest pay sands are fully exploited. Considering the development from each individual platform, the EC Block 265 field followed this general rule. After development drilling ceased on Platform A and two initial Platform B wells were drilled, Texaco elected to use a 3D seismic survey to evaluate the potential of additional fault blocks and deeper reservoirs in the EC Block 265 field. Platform B, set in 173 ft of water, was strategically placed to avoid penetrating shallow gas hazards en route to deeper objectives. The major difficulty of this development plan was to drain the shallow gas reservoirs properly without causing significant water coning. A horizontal or near-horizontal well was determined to be the best way to open up sufficient reservoir drainage footage and to reduce or eliminate
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