A New Effective Stimulation Treatment for Long Horizontal Wells Drilled in Carbonate Reservoirs

Abstract

Effective matrix acidizing of horizontal and multilateral wells can be a highly challenging task. Unlike vertical wells, horizontal wells can extend several thousand feet into the formation. Reservoir heterogeneity and the length of the horizontal leg can make acid placement and diversion very difficult. In addition, the low drawdown encountered in horizontal wells results in longer times to lift the spent acid from the well, especially in tight formations.To achieve better acid diversion in horizontal wells drilled in carbonate reservoirs, a viscoelastic-surfactant-based system was used. The components of this new system are HCl and a viscoelastic surfactant. The acid dissolves calcite and dolomite minerals and produces calcium and magnesium chlorides. The increase in pH forces the surfactant molecules to form rod-shaped micelles. The produced chloride salts further stabilize these structures, especially at high temperatures. The rod-shaped micelles will significantly increase the viscosity of the acid, diverting the acid into tight, unstimulated, or severely damaged zones.More than 100 wells with openhole (OH) completions were successfully stimulated in two offshore oil fields in Saudi Arabia by use of the new acid system. With a water zone 30 ft away from these OH sections in one of the fields, growth of any dominant wormhole into these sections could increase water production. The wells that used the new treating fluid produced an average of 1,600 BOPD more than conventionally treated wells, with no indication of water production. Field results [pre-and post-oil and -water production rates and flowing wellhead pressure (FWHP)] demonstrate the effectiveness of the new acid system to matrix acidize long horizontal wells with OH completions. The simplicity of the system makes it the fluid of choice, especially in offshore and sour environments. The absence of metallic crosslinkers in this system eliminates problems associated with sulfide precipitation in sour wells.

A Novel Technique to Acidize Multilateral Open Hole ER Horizontal Wells Drilled in Carbonate Formations

“…The surfactant used was amphoteric and the rheological properties and field cases using this system were given in previous publications. [24][25][26][27][28] Tables 3 and 4 give the acid formulations used. The acid volume used in both wells was 10 gals/ft of open hole.…”

“…21,22 Positive field results were reported when this acid system was used to stimulate oil producers, water injectors and water disposal wells. [23][24][25][26] Recently, viscoelastic surfactant-based acids were used to stimulate OH sections that could not be reached because of CT lock-up limitations. Openhole sections of up to 2,000 ft were successfully stimulated by bullheading the treating acids at the CT lock-up depth.…”

“…Openhole sections of up to 2,000 ft were successfully stimulated by bullheading the treating acids at the CT lock-up depth. 26,27 A major advantage of viscoelastic surfactant-based acids over conventional gelled acids is that they do not contain polymers and hence they are less damaging. Unlike other diverting materials, such as foam and particulates, viscoelastic surfactant-based acids can be pumped as a single fluid, which will stimulate and divert at the same time.…”

“…This fluid proved to be very successful in bullheading operations for long perforated intervals 24 and short OH horizontal sections. 26,27 The in-situ stimulation and self-diverting properties of the viscoelastic surfactant-based acid system made it the fluid of choice in this application. The diversion capability of the fluid tends to extend the reach of acid coverage to other non-stimulated sections.…”

A Case Study of a Successful Matrix Acid Stimulation Treatment in Horizontal Wells Using a New Diversion Surfactant in Saudi Arabia

“…6,7 To overcome some of the problems encountered with polymer-based fluids, viscoelastic surfactant-based acid systems have been successfully used over the last few years. [8][9][10][11][12][13][14] Simple inorganic salts cause the molecules of viscoelastic surfactants to form long worm-like or rod like micelles. 15 Entanglement of these micelles generate a 3-D structure in solution which increases the viscosity of the solution.…”