Schlumberger

Technical Paper: The Introduction of an At-Bit Natural Gamma Ray Imaging Tool Reduces Risk Associated with Real-Time Geosteering Decisions in Coalbed Methane Horizontal Wells

Society: SPWLA
Paper Number:
Presentation Date: 2012
 

Abstract

Logging While Drilling (LWD) measurement selection is an exercise in risk management where potential drilling hazards with or without a specific measurement are weighed against their benefits. The assessment compares value added to real-time steering decisions for the area specific character of the target reservoir, mechanical limitations of the drilling and completions assemblies and economics. LWD borehole images are commonly used to place complex, long reach lateral wells into the optimal section of the reservoir. The major advantage of the use of images is that they indicate the direction in which bed boundaries are crossed (top-down or bottom-up). Additionally, the bed dips can be calculated which aid in constructing a realistic structural model of the formations of interest. Typically, the LWD tools, including the imaging measurement(s), need to be positioned in the bottomhole assembly (BHA) above the mud motor or rotary steerable system (RSS). This results in a measurement point that is some 40 ft to 80 ft behind the bit. Considering the relatively shallow depth of investigation of most imaging measurements, the images do not represent the current position of the bit in the stratigraphy - the bit may already have drilled out of the zone before this is identified on the images. A real-time at-bit natural gamma ray imaging tool was developed as a solution to position the image measurements as close to the bit as possible to reduce the reaction time for time critical geosteering decision making processes.

The new tool consists of two separate subs. An instrumented stand-alone sub containing the measurement sensors is located directly above the bit. A wireless telemetry system is used to transmit the data across the mud motor to the second sub which is connected to the bottom of the conventional MLWD toolstring. The at-bit measurements are then transmitted to surface via conventional mud pulse telemetry. The real-time at-bit measurements consist of a natural gamma ray curve, eight sector natural gamma ray image and dynamic inclination. 

The tool has been extensively used in the drilling of coalbed methane wells in a complex folded and faulted environment in laterals up to 5000 ft long. The coal seam targets are selected based on offset well log data. The character of gamma ray readings for the targeted coal is evaluated along with the density measurements to generate the initial geosteering model(s) for the well to be drilled.  While drilling, the geosteering model is continuously updated with the at-bit gamma ray measurements and the dip of bedding planes is computed. In a challenging geological structure with the rate of penetration that can commonly exceed 100ft/hr, multiple steering decisions may be required every hour in order to position the well in-zone. Deciding on the correct course of action after crossing a fault can be particularly challenging – is the fault upthrown or downthrown? At-bit gamma ray images have enabled a repeatable decision making process to maintain well position within the coal seam where every foot in zone translates into revenue for the operator.

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