Thursday, March 21, 2019

Controlling Fluid Loss

A portion of the fluid pumped during a fracturing treatment filters into the surrounding permeable rock matrix. This process, referred to as fluid leakoff or fluid loss, occurs at the fracture face.  The volume of fluid lost does not contribute to extending or widening the fracture. Fluid efficiency is one parameter describing the fluid's ability to create the fracture. As leakoff increases , efficiency decreases. Excessive fluid loss can jeopardize the treatment, increase pumping costs and decrease post-treatment well performance. 

Typically, particulates or other fluid additives are used to reduce leakoff by forming a filter cake- termed an external cake- on the surface of the fracture face. Acting together with the polymer chains, the fluid-loss material blocks the pore throats, effectively preventing invasion into the rock matrix.

 This approach has been applied successfully for decades to low-permeability (< 0.1 md) formations in which polymer and particulate sizes exceed those of the pore throats. In high permeability reservoirs, however, fluid constituents may penetrate into the matrix, forming a damaging internal filter cake. This behavior has prompted mechanistic studies to determine the impact on fracturing treatment performance.

 Classic fluid-loss theory assumes a two-stage, static - or nonflowing-process. As the fracture propagates and fresh formation surfaces are exposed, an initial loss of fluid, called spurt, occurs until an external filter cake is deposited. Once spurt ceases, pressure drop through the filter cake controls further leakoff. For years, researchers have developed fluid-loss control additives under nonflowing conditions based on this theory.

 The conventional assumptions, however, neglect critical factors found under actual dynamic - or flowing- conditions present during fracturing, including the effects of shear stress on both external and internal filter cakes and how fluid-loss additives move toward the fracture face. In high-permeability formations, with an internal filter cake present, most of the resistance to leakoff occurs inside the rock, leaving the external cake subject to erosion by fluid.

 Analysis of fluid loss under dynamic conditions relates external cake thickness to the yield stress of the cake at the fluid interface and the shear stress exerted on the cake by the fluid. These, in turn, depend on the physical properties of the cake and the rheological properties of, and shear rate induced in , the fluid. Whether an external filter cake forms, grows, remains stable or erodes depends on the way these parameters vary and interact over time and spatial orentation.

Similarly, the effectiveness of additives to control fluid depends on two factors: their ability to reach the fracture face quickly and their ability to remain there. The former is governed by the drag force exerted on the particles and the latter by the shear force exerted on them. The larger the ratio of drag to shear , the greater the chance that the particles will remain on the surface. A greter leakoff flux to the wall, smaller particle dimensions and a lower shear rate favor sticking. Promoting higher leakoff for better additive placement seems directly at odds with controlling fluid loss! However, in practice, higher initial leakoff can yield greter overal fluid efficiency. 

To confirm the controlling mechanisms, dynamic fluid-loss tests were conducted  using a slot-flow geometry, determined to be the simplest representation of what occurs in a fracture. To completly describe the process, computer-controlled equipment was constructed to prepare and test fluids under dynamic conditions, subjecting them to the temperature and shear histories found in a fracture. Cores of various lengths were used in the tests to simulate a fracture segment at a fixed distance from the wellbore. As the fracture tip passes a spesific point, spurt occurs and the shear rate reaches maximum. Then, as the fracture widens, the shear stress decreases. In the test apparatus, this is stimulated by decreasing the flow rate with time. Pressure sensors along the core monitor the progress of the polymer front.

Laboratory tests show that , for comparable fluids and rocks with permeabilities of up to 50 md, fluid loss is greater under dynamic conditions than static conditions. Further, examining the impact of shear stress and permeability on the magnitude of fluid loss and the effectiveness of leakoff control additives in high-permeability formations led to five key conclusions.

First, high shear rates can prevent the formation of an external filter cake and result in higher than expected spurt. Second, an internal filter cake controls fluid loss, especially near the fracture tip. Third, the effectiveness of fluid-loss additives increases with formation permeability and decreases with shear rate and fluid viscosity. Fourth, reducing fluid loss means reducing spurt, particularly under high shear conditions and in high-permeability formations. 

The effect of shear depends on the type of fluid and the formation permeability. Typically, above a threshold shear level, no external cake is formed. The magnitude of fluid loss is dependent on the type of polymer and whether it is crosslinked. If the permeability is high enough and the fluid structure degrades with shear, polymer may be able to penetrate the rock matrix.

Dynamic test revealed that commonly used additives were less effective in controlling fluid loss than static test had previously indicated. Also, a direct link between fluid efficiency and shear rate was demonstrated. The higher the fraction of fluid lost under high shear early in the treatment, the higher the total leakoff volume and the lower the efficiency.


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