Horizontal wells have shown such gains in productivity in many applications that the damage associated with longtime exposure to drilling fluid was, in many cases, accepted. In addition, the dif-ficulty of removing formation damage in a horizontal well has compounded the problem. With many horizontal wells now being left in an openhole status, formation damage becomes even more important.

Coiled tubing (CT) drilling has grown from four jobs in 1991 to over 120 (estimated) jobs in 1994. The primary motivations for this growth have been:

  • The ability of CT drilling to finish drilling a well in soft forma-tions faster than a rotary rig, and

  • Safe, rigless underbalanced drilling to greatly reduce forma-tion damage in horizontal wells.

This paper reviews the causes of formation damage, both from fluid/solids invasion and stimulation techniques to remove dam-age.

An analytical model is used to estimate the productivity index (PI) for various horizontal and vertical permeabilities, well lengths, and reservoir thicknesses for comparison with results from several case studies.

Options for underbalanced drilling including fluid selection, gas lift, and seal technology are discussed. Candidate selection cri-teria for those evaluating the possibility of underbalanced hori-zontal drilling are presented.

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