Abstract

High-pH, guar-based fluids are favored in fracturing treatments for their low cost, ability to recover viscosity after mechanical shearing, and favorable environmental properties. Seawater can be considered as the base fluid when fresh water availability is limited. However, seawater presents a technical challenge for such fluids due to copious precipitation at high pH. The goal of this study is to find additives that control divalent cation precipitation to enable a seawater-based borate fracturing fluid suitable for high-temperature application, ideally with the additional benefit of long-term scale control.

Complete sequestration of cations with stoichiometric amounts of citrate or EDTA is not economically or logistically feasible. Instead, a crystal modifier approach was adapted to control precipitation of divalent cations, especially Mg2+. An additive has been identified to prevent precipitation of divalent cations at high pH, as required for high-temperature borate fluids. Fluids with this additive perform as well as those formulated in fresh water. The new technology further minimizes long-term scaling risks of divalent cations in the fluids in conjunction with formation water.

Conventionally, medium pH was achieved with amines to avoid severe precipitation of divalent cations in seawater. As a result, such approach is only used in relatively low-temperature applications. Low-pH fluids do not function well at high temperatures due to facile acidic hydrolysis. A versatile additive, capable of controlling divalent cation precipitation and providing long-term scale control, has been identified. Compared to citrate, EDTA salts, and other commercial additives, the new additive shows superior divalent cation precipitation control capability in seawater even at high pH, at significantly lower concentrations in lab bottle tests. Borate fluids prepared in seawater with the new additive showed excellent rheological properties at temperatures above 300°F on a Chandler 5550 HPHT rheometer. The additive is compatible with fluid additives and is easy to apply. Dynamic tubing-blocking tests showed that this chemical is also highly effective in controlling anhydrite scale in seawater-based fluid mixed with formation water that contains more than 20,000 ppm of calcium scaling ions. Proppant conductivity testing confirmed the fluid can be effectively cleaned up, leaving minimal proppant pack damage. This additive offers the most economical solution to formulating fracturing fluids with seawater and with minimal scale risk.

The study presents a new additive that meets both technical and economic requirements to enable effective high-temperature hydraulic fracturing and minimizing scale risks in areas where adequate fresh water is not readily available or is limited by environmental regulations. Rheology, conductivity, and tubing-blocking test results demonstrate that seawater can be used as a base fluid for fracturing treatments in high temperature, by effectively controlling divalent precipitation approach which also offers a longer-term advantage of scale control.

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