This paper presents and discusses guidelines for matrix stimulation design, real-time control and post treatment evaluation on the basis of field results obtained from eight (8) horizontal and four (4) drain hole wells. A stimulation technique originally developed for vertical wells1 , has been successfully modified and applied to horizontal wells. An optimized treatment procedure consists of placing the acid mixture downhole through coiled tubing and bullheading "ad hoc" volume of treating fluid at maximum bottom-hole pressure differential and rate. Equations to link measurable injection parameters (pressure and rate) with evolving skin effect are presented. In order to evaluate the effectiveness of the stimulation treatment, several Production Logs were run downhole using a coiled-tubing-conveyed wireline system. These revealed a remarkable distinct relationship between the producing horizontal length and the injection rate. A stimulation rate as high as 52 bbl/min (pumped in matrix, without fracturing, conditions) allowed almost total (95%) horizontal length contribution to the overall production. Without the use of diverting the technique was applied effectively to long horizontal production intervals (proven up to 1500 ft max).

The most effective results from this stimulation technique are obtained when dealing with reasonably homogeneous primary permeability reservoirs. For macrofractured formations (which are generally excellent candidates for horizontal wells) mechanical isolation plus the present technique is a more successful stimulation method to adequately cover the entire production interval.

Case histories are also described. A case of a stimulation repeated in the same well using different procedures is reported, as well as features of Production Logging Tools used in the field.

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