The low permeability carbonate reservoir in central Iraq was discovered in the 80's, facing the acid stimulation demand of shut-in well reactivation and new drilling well high-efficiency production. Low permeability, poor pore connectivity and non-ideal production necessitate the shift from conventional acidizing and pad fracturing to integrated multistage acid fracturing techniques, which achieve better fracture conductivity and deep penetration to maximize the production potential.

Petro-physically speaking, effective porosity, net pay and hydrocarbon pore volume are related to reservoir performance. In this low permeability carbonate reservoir, previous acid stimulation analysis shows that the factors affecting production capacity are far more than those above mentioned. The injection parameters, surface etching and conductivity were lab-tested and evaluated among the four techniques of matrix acidizing, pad fracturing, alternating injection with acid and linear gel and proppant-slurry acid fracturing. The proppant-slurry acid fracturing was the first-ever proposed to unlock the production potential. Comparing to the conventional acid stimulation, the parameters including the proportion of crosslinked gel and acid, concentration, size and total volume of proppant, multistage injection volume and rate, retardance and closed acidizing, have been optimized and designed successfully for the first pilot well on-site.

After the lab tests, geomechanics and modeling and parameter simulation of proppant-slurry acid fracturing, the results show that the stress difference between the perforated layer and the upper rock layers is about 5 MPa, the geometry of the artificial fractures can be controlled in between oil layers. Water-based guar gel fracturing fluid with additives and 20% concentration HCL with retarded agents were selected, the maximum pumping rate was designed as 6.0 m3/min with 6862 Psi pumping pressure, and the volume of acid was double of fracturing fluid. After the pumping of 20 m3 20/40 proppant with maximum concentration of 583 kg/m3 and 400 m3 acid, the evaluation results showed that the propped half-length Xf = 116.4 m width Wf = 0.7 cm, and dimensionless conductivity is 2.6. The predicted production was 2.5 times greater than before stimulation, and the artificial fracture was initiated in the pad stage, and then a finite conductive channel was formed by dual support of surface etching and proppant.

Many technical and operational challenges were faced and properly handled with lab tests, numerical simulation and evaluation, which will form the completion and stimulation strategies for the low permeability carbonate reservoir. Four jobs have been confirmed for proppant-slurry acid fracturing.

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