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For the problem of low success rate in the communicating operation of CBM horizontal well drilling operations using MWD, the rotating magnetic locating tool, MTS, was developed, which can supply powerful supplement and calibration for MWD measurement within a certain distance. The paper described the technical process and role of the communication. Beginning with the measuring principle of MTS, the measurement errors of the two tools were compared in three aspects: measurement lagging, working style, and error propagation and error ellipsoid. Field examples of the successful application of MTS in the CBM connectivity operations were described. The result can be obtained that different measurement principle and different working way cause different error propagation, thereby lead to the difference on measurement accuracy and communicating job success rate. [ABSTRACT FROM AUTHOR]

Abstract: To reduce the current high completion cost for steel slotted screen pipes in coalbed methane wells and to develop low-intensity non-metal completion pipes to perform low-cost completion, this paper adopts electric survey to measure the collapsing strength and bending strength of J55 steel screen pipe and PVC non-metal screen pipe, and makes a comparative analysis of the effect of slot width and slot density on the strength of both kinds of screen pipes. The research results indicate that: the collapse of steel screen pipes mainly results from stiffness reduction or radial unstableness, showing the whole pipe deformation or the slot becoming wide or narrow at the internal or external wall along the short axis, accompanied with the tearing at the slot end-point; while the PVC screen damage mainly results from the material itself. For both steel and PVC screen pipes, the pipe''s collapsing/bending strength declines with increase of the slot width and density, but the PVC screen pipes do not show such an obvious trend, making it possible to apply wider and denser slots on PVC pipes to increase the passing area. [Copyright &y& Elsevier]

Abstract: When horizontal wells are drilled underbalanced in coal seam, it was found that borehole which kept stable at the beginning would probably collapse in some time. The phenomenon was called as time delay effect. In order to clarify the concept of time delay effect, the influence of pore pressure on stresses of cleats was analyzed based on the characteristic that cleats are abundant in coal seam, indicating that pore pressure influences the normal stresses of cleat surface while it has no impact on the tangential stresses. Using the data from field production in a CBM well, the influence of pore pressure changes on stability of coal rock was analyzed, indicating that contents of coal fines in drained water increases as pore pressure decreases. Pore pressure changes due to seepage in underbalanced drilling or overbalanced drilling were analyzed. This study above demonstrates that the time delay effect exists absolutely in borehole stability in coal seams during underbalanced drilling. The pore pressure changes are the main factor on the time delay effect, while cleats are the internal cause. In order to analyze further time delay effect quantitatively, the fluid–solid coupling during underbalanced drilling was numerically simulated, and the relationship between pore pressure and time was fitted, and the equation for computing the delaying time was posed by means of borehole stability theory in fracture mechanics. The analysis of the time delay effect can guide the field drilling operation and avoid the instability caused by pore pressure changes. The delaying time can be altered through adjusting bottom hole pressure in order to supply sufficient time for drilling operation. [ABSTRACT FROM AUTHOR]