Kotabatak field, Sumatra, Indonesia is a heavily-faulted field undergoing an aggressive drilling and development campaign. Nine horizontal wells had been drilled with four more planned in 2008. One of the horizontal wells recently experienced well collapse (and sudden productivity decline) after some time on production, with cavings being flushed out during coil tubing workover operations. In addition to horizontal well drilling, feasibility of open horizontal well completions, hydraulic fracturing design and sanding onset prediction also warranted rock mechanics analyses. To make sound decisions on those issues, building a well-calibrated geomechanical model was critical. In this study, we reviewed the drilling, completion, logging and production information from several wells across the field. We found that (1) The Kotabatak field has a general maximum horizontal stress orientation of NESW. However, there could be localized stress orientation variations depending on structure complexity near a specific well. (2) There was no consistent evidence indicating a significant contrast between the maximum and minimum horizontal stresses. Using a maximum/minimum horizontal stress ratio of 1.05 yielded a consistent calibration result for the wells studied. (3) Sand minimum horizontal stress for the Kotabatak field was calibrated against available closure stresses from hydraulic fracturing and mini-frac data. (4) Rock mechanical properties were calculated with openhole logs based on a Rock Mechanics Algorithm that is closely linked to Chevron's worldwide rock mechanical property database. Consequently, even though there were no core test data available from the Kotabatak field to calibrate rock mechanical properties directly, the log data set provided the means to estimate reliable formation mechanical property values that are consistent with Chevron's worldwide database. Furthermore the entire geomechanical model was calibrated against offset drilling performance measures resulting in a high degree of confidence in the predicted values. Using the calibrated geomechanical model, horizontal well stability predictions were performed and indicated that horizontal sections can be drilled with low mud weight allowing the well to have some yield/failure. Open horizontal well sanding onset prediction indicated that the depth and width of a breakout (or plastic zone if reservoir sand behaves plastically) increase with increasing pressure drawdown. Since water flooding is used in the field to maintain reservoir pressure, sand control may not be needed if an appropriate Bottomhole Flowing Pressure (BHFP) is applied. Introduction The Kotabatak field, Sumatra, Indonesia is a heavily faulted field undergoing an aggressive drilling and development campaign ((Figures 1 and 2). Nine horizontal wells had been drilled (as of the end of 2007) with four more planned in 2008. One of the horizontal wells recently experienced well collapse (and sudden productivity decline) after some time on production, with cavings being flushed out during coil tubing workover operations. In addition to horizontal well drilling, feasibility of open horizontal well completions, hydraulic fracturing design and sanding onset prediction also warranted rock mechanics analyses. To make sound decisions on those issues, building a well-calibrated geomechanical model was critical.
Three Vertical Seismic Profiles and one well shoot mere acquired as pari of the EWDP-A revisit-3 horizontal drilling campaign in the Erb West field. The main objective in acquiring this data was to derive a reliable velocity model for the overburden in the main part of the field. By positioning the geophones in the horizontal section, and below the heavily faulted overburden, allowed for correct imaging of reservoirs immediaiely belozu the wellbore and the dipping horizons that subcrop the Shallow Regional Unconformity. Also an attempt will be made to integrate the VSP rcsults with the newly acquired surface 3D seismic to help identify multiple patiems, and to guide the derivation of velocities for stacking and migration. The acquisition involves the deployment of, a 3- component, gimbal-mounted seismic geophone that is drill pipe conveyed into the well. As the tool is pushed into the hole, it is siopped at pre-determined stations, rigidly coupled to the casing, and acoustically isolated from the drillpipe to provide a good signal-to-noise ratio. The seismic source is positioned directly over the sensor, and a serie.s of shots are recorded at euch Station. The main problem with processing VSP data acquired in a horizontal well is the Separation of the upgoing from the downgoing wavefields. In conventional VSP Processing this is accomplished by the geophone being vertically displaced in the wellbore. In the Erb West field, this was accomplished by using the downgoing wavefield obtained over the build up section together with differences in moveout betzoeen the direct arrivals and reflected events from structurally tilted reflectors beneath the horizontal part of the hole. This all ozo ed for the creation of a high resolution seismic image in the vicinity of the well.
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