Matrix Acidizing in Saudi Arabia Using Buoyant Ball Sealers
Authors: Bale G.E. 
Published: December 2007

Acid jobs utilizing buoyant ball sealers have been performed on several wells with varied downhole conditions and different productivities. The test objectives were to determine buoyant productivities. The test objectives were to determine buoyant ball sealer's diverting efficiency when pumping at matrix acidizing rate and to determine whether or not the buoyant ball sealers are more effective than the particulate diverting materials presently being used. Complete pre and post-acidization testing provided accurate data for valid test analysis.

The buoyant ball sealers were effective in every test. Matrix acidizing of long intervals and multiple sets of perforations was successfully accomplished. Marked production perforations was successfully accomplished. Marked production improvement resulted from a well previously acidized utilizing particulate diverting materials. Production of an old well particulate diverting materials. Production of an old well previously acidized six times was increased to the highest rate in previously acidized six times was increased to the highest rate in its sixteen year producing history.

Buoyant ball sealers are the most effective means of diverting acid when matrix acidizing perforated completions.

Introduction

Stimulation in Saudi Arabia is performed primarily to remove the perforation damage and to establish a flow path through formation damage near the wellbore. Completion intervals in our carbonate reservoirs may be either open hole or perforations. Intervals will vary from 10 to 90 m (30 to 300 ft) or more in thickness with diverse porosities and permeabilities. Various particulate diverting materials, wax beads, benzoic acid flakes, particulate diverting materials, wax beads, benzoic acid flakes, and graded rock salt have been used with varying degrees of success. High density ball sealers, 1.3 gm/cm have been used.

There has been problems with each diverting system. Determination of the proper quantity of particulate diverting material to use is difficult. Insufficient diverting material will perform little or no diverting. Too much diverting material will cause excessive plugging which may not clean up and will leave the formation damaged. An example of excessive plugging occurred in an exploration well and illustrates the problem. Eleven meters (35 ft) of a reservoir were perforated, acidized, and were production tested for 160 m/d (1000 BOPD). Fourteen meters (45 ft) more of the reservoir above this were perforated. Rather than setting a bridge plug or squeezing the interval with cement, particulate diverting material was added to the first portion of the acid used for stimulating the new perforations. portion of the acid used for stimulating the new perforations. With both sets of perforations on production test together, the well produced only a small quantity of oil. The original production was not recovered.

Diversion of treating fluids generally result from the use of high density ball sealers whenever high injection rates are maintained and the perforation density has been limited to 2 shots per foot. The minimum injection rate required for good per foot. The minimum injection rate required for good perforation sealing efficiency using high density ball sealers is perforation sealing efficiency using high density ball sealers is greater than calculated matrix acidizing rates for most wells. When not limited to matrix acidizing, the injection rate required for effective diverting with high density ball sealers is not possible because of the large friction losses in our small tubing possible because of the large friction losses in our small tubing kill strings and the pressure limitations of our well heads. In most acidizing jobs we have performed utilizing the 1.3 gm/cm ball sealers, diverting efficiency has been low because of low injection rate and the perforation density of 4 shots per foot.

Exxon Production Research Company recognized the problems associated with particulate matter diverters and the high density ball sealers several years ago and undertook a study to improve ball sealer diverting efficiency. Their study revealed that buoyant ball sealers exhibited a very high seating efficiency resulting in greatly improved diversion.

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