The reserves of tight carbonate S formation amounts to 25% of the total of X field in Iraq, but the production contributed only 0.3% to the total output. Accordingly, hydraulic fracturing was implemented in well V5X in Dec. 2016, the first pilot test in Iraq. In this paper, the fractured well was taken for a case study to illustrate the holistic optimization to guarantee the treatment success, a world-wide difficulty with high engineering risk.

There are uncertainties concerning the breakdown pressure, in-situ stress and the development of natural fissures in the region, posting challenges in figuring out a reasonable treatment design.

For this purpose, comprehensive lab experiments and numerical simulation were conducted to analyze and benchmark the reservoir characteristics, rock mechanics and geological model. Systematic study on fracture parameters optimization, treatment fluid formulation, proppant screening and operation program were carried out. Based on the treatment parameter optimization, the main-frac scheme was perfected in combination with mini-frac and temperature logging interpretation.

S formation is a limestone reservoir with unique petrophysics, i.e., medium to high porosity (15-20%) but low permeability (0.05-1.3 mD), poor connectivity between pore throats, and hence resulting in inadequate liquid supply. Lab experiments revealed the "low Young's modulus and high Poisson's ratio" rock mechanics features, which is contrary to common understanding. In this case, large-scale treatment was required to ensure the high fracture conductivity. Moreover, there is no significant contrast in the in-situ stress profile or lithology barrier. Consequently, the development plan which requires fractures to be constricted within S formation would be hardly realized. Therefore, proppant sizing was prudently conducted to achieve the rational sand placement and treatment fluid performance was optimized to control the vertical fractures extension. For the fracturing job, totally 443.1 m3 treatment fluid and 39.6 m3 proppant were pumped. The operation parameters were in reasonable agreement with the design. The initial test production was 10 times higher than before. Currently the "production under controlled pressure" mode has been executed. The wellhead pressure has been stable, and the boosted production has been maintained for over one year. Comprehensive analysis of reservoir characteristics and suitable fracturing design are crucial to the success of stimulation treatment.

The experience gained in this case study has some guiding significance for improving the success rate of hydraulic fracturing treatments and for effectively developing such low permeability carbonate reservoirs, both theoretically and practically.

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