Successful acidizing techniques have been developed andimplemented in the Main Pass area, offshore Louisiana. Since mid-1984,37 oil wells have been acidized. resulting in a success rate of 95% anda total increase in production of almost 12,700 BOPD [2019 m3/d oil].
One of the primary reasons these treatments were so successful wasthe continuous injection of a finely ground oil-soluble resin divertingagent throughout all acid stages, which decreased acid injection into themore permeable zones and helped distribute the treating fluids moreuniformly.
Other factors that may not be the industry norm that influenced theeffectiveness of these treatments included (1) reduced-strength HCl/HF andpreflush acids, (2) xylene solvents pumped ahead of injected acid for preflush acids, (2) xylene solvents pumped ahead of injected acid for possible paraffin and/or asphaltene removal, (3) xylene pumped after the possible paraffin and/or asphaltene removal, (3) xylene pumped after the overflush stage as a diverter breaker, (4) filtration of all fluids, (5)low-rate, high-pressure pump for continuous injection of diverterdownstream of filters, (6) use of "pickled" coiled tubing, (7) coil/production tubing annulus pressure recorder, (8) testing production tubing production tubing annulus pressure recorder, (8) testing production tubing before pumping, (9) use of boats exclusively, rather than offloaded skids, from which to mix and pump, (10) on-site acid mixing with titration checksof concentration before pumping, and (11) postjob laboratory analysis ofconcentrations and solids content of selected spot samples.
The Main Pass wells acidized are located offshoreLouisiana in Blocks 288, 296, and 311, about 30 miles [48km] east of the mouth of the Mississippi River. Thirteendifferent sands, which range in depth from 4,500 to 6,500ft [1372 to 1981 m], have been acidized to date. Thenative permeabilities of these formations range from 100to 1,000 md. Other average reservoir parameters include27% porosity, 25% water saturation, and 140 degrees F [60degrees C] formation temperatures. Perforated intervalsrange from 12 to 110 ft [4 to 34 m], and the majority ofthe wells are dually completed.
Of the wells acidized in this program, when completed,51 % were perforated overbalanced with 16 shots/ft [52shots/m] followed by perforation washing. The other 49%were perforated with 12 shots/ft [39 shots/m] with variousdegrees of underbalance. All completions weregravel-packed for sand control, and all completion fluid wasfiltered. Both types of completions produced some wellswith high degrees of skin damage.
The formations acidized contain low quartz, high clayminerals, high feldspar, iron minerals, andlow-API-gravity crudes (15 to 28 degrees API [0.97 to 0.89 g/cm3]), which are undesirable characteristics in sandstone acidcandidates. This rock composition, combined withsand-control installations and completion fluid, is oftensusceptible to impairment, which can be chemical (such as claydamage) or mechanical (such as particulate pluggingcaused by entrained solids in the completion fluid) innature.
Skin damage is a major reason many Main Pass wellsdo not reach their production potential. Determiningwhether the major cause of damage is drilling, perforating, gravel packing, or migrating fines is very difficult, perforating, gravel packing, or migrating fines is very difficult, however, each of these probably contributes to someextent. This acidizing technique was designed to increaseor to restore permeability into the wellbore with aminimum disturbance of the stabilized wellbore inflowframework by reducing skin.
Few solid correlations were found between success(i.e., skin reduction) and any one of the new techniquesperformed because so many techniques were changed at performed because so many techniques were changed at once that no single item could be independently tested. Because we think that each new technique has theoreticaland practical merits, we were reluctant to reduce theeffectiveness of the job by omitting one or more for the sakeof science.
Our data base is relatively small and confined to arelatively narrow, type of formation, but the techniquespresented are sound. and most are applicable to all presented are sound. and most are applicable to all sandstone acidizing.
Since the installation of our first Main Pass platform in1970. acid stimulation has been used to restore andmaintain production rates from wells in decline. Threedistinct acidizing periods that contributed to the evolution of ourcurrent design were experienced: pre- 1978, 1978–81, and1982–84. These periods are evidence of advancingtechnology in the industry. as well as changing management philosophy.