Calcium sulfate is inherently a difficult mineral scale during oil and gas production process because the amount of scale formed is much greater than that of barium sulfate at similar scale saturation index level, and it is very difficult to clean up. This is especially challenging in conjunction with HTHP stimulation treatments where compatibility of the scale control chemical with fracturing fluids is critical, and when longer-term inhibition performance is desired. A new solid inhibitor was developed for this purpose and applied in multiple wells in the Krishna Godavari (KG) basin offshore India to combat mineral scale within the proppant pack and production tubing over the long term, under extreme downhole conditions (T= 400°F, P=13,500 psi). Normally, downhole chemical injection mandrels and surface treatments cannot adequately control scale deposition under these conditions.
The new solid inhibitor product was made by adsorbing scale inhibitor onto a high-strength, proppant-sized substrate with a large surface area. The high-strength substrate were prepared by sol-gel chemistry through hydrolysis of aluminum alkoxides and formation of particles that are calcined and then sintered at high temperatures to produce a substrate with the desired strength and surface area. The scale inhibitor used exhibited excellent inhibition performance and good compatibility with metal based cross-linked fracturing fluid systems at high temperature.
Tests performed with proppants/substrates show that using high loading of the substrates with the proppant does not damage the proppant pack even under very high stresses, For example, API crush tests of a mixture of 80% conventional untra-high strength proppant with 20% substrate by weight at 13,000 psi produced less than 4.7% fines and 88% of the produced fines were larger than 100 mesh and the fracture conductivity of the pack is maintained. The results of comprehensive laboratory testing show the new solid inhibitor can prevent anhydrite scale up to 400°F, and is completely compatible with zirconium- crosslinked fracturing fluid at 350°F and above. To date, six fracture treatments have been performed using a total 23,800 lbs of this new solid inhibitor. The wellhead water samples are being collected for scale inhibitor residuals analysis, as the wells start to produce water.
To ensure compatibility of the inhibitors with high-temperature fracturing fluids, especially metal based cross-linked fracturing fluids, without compromising the inhibition longevity at high pressure and temperature remains a stiff challenge, although adding scale inhibitors to a fracturing fluid has been a well-established practice to provide long-term inhibitor protection during hydrocarbon production. The new approach described here meets this objective, extending the long-term well performance under HTHP conditions.