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 difficulty 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 formations faster than a rotary rig, and
Safe, rigless underbalanced drilling to greatly reduce formation damage in horizontal wells.
This paper reviews the causes of formation damage, both from fluid/solids invasion and stimulation techniques to remove damage.
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 criteria for those evaluating the possibility of underbalanced horizontal drilling are presented.
Horizontal wells are particularly vulnerable to formation damage due to long drilling mud exposure time relative to vertical wells (easily 30 times) and reduced cleanup velocity (easily 1/5) with production spread over the long horizontal. In spite of this, horizontal wells are the most important success story in the oil business in the last 10 years.
Average gains in productivity1,2,3,4 of two to seven times the vertical wells in reservoirs with matrix permeability have fueled the growth of horizontal drilling. In reservoirs with natural fracture, productivity indices of 20 to 30 times the vertical have been observed with ultimate recovery several times greater than for vertical wells.5 Ultimate recovery improvement with matrix permeability is not widely reported, although 1.5 to 2 times vertical ultimate recovery is expected in two Canadian reservoirs. Although the ultimate recovery may not be improved, the accelerated production often makes horizontal wells more economically attractive in spite of the higher cost of the average horizontal well which is 1.2 to 2 times the vertica.3
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