Modeling Fracture Tip Screenout and Application for Fracture Height Growth Control

Abstract

This paper discusses the use of numerical simulations for predicting the onset and development of fracture tip screenout (TSO). It presents a parametric study for fracture containment to prevent height growth into water boundaries or a gas cap, under conditions that approximately simulate Gulf of Mexico reservoirs (weakly consolidated, high permeability soft formations). Different typical field cases were analyzed to demonstrate height containment at TSO conditions. The perforation interval is critical to height containment, with perforation height of only a fraction of the total pay interval required in some cases in order to create TSO. Introduction As background, this section summarizesunder what conditions a TSO occurs;how the planar three-dimensional simulator TerraFRAC™, used in this analysis, models that mechanism; andhow simulations can provide an understanding of the mechanisms occurring during the frac and pack operation. A typical TSO treatment consists of pumping a volume of proppant free fracturing fluid (pad) followed by a volume of proppant laden slurry. The pad, although small in volume, may develop most of the final fracture geometry. The slurry upon entering the fracture dehydrates due to rapid leakoff of the fracturing fluid and ‘packs’ the fracture with proppant - particularly near the fracture tip. The packed fracture creates a pressure drop along the fracture that eventually restrains and/or arrests fracture growth once most or the entire fracture tip is packed; i.e. a tip screenout. When this happens the net pressure near the well bore increases, usually rapidly. Further pumping results in inflation of the fracture, and eventually in some cases to fracture extension and a corresponding reduction in net frac pressure. If proppant dehydration via leakoff is effective - particularly near the fracture tip - packing of the new fracture area will again occur with another pressure build up results. If during the process the fracture breaks into a low leakoff formation (i.e. breaks into the overlying shale for example), dehydration likely will not occur and the fracture will grow unlimited without the occurrence of a TSO. Net pressure will generally continue to decrease.1–3 The planar three-dimensional simulator for this analysis uses a progressively refined and dense mesh of small triangular elements that allow true calculation of the fracture geometry based on fracture mechanics that advances the crack. Proppant transport is calculated as well as leakoff into the formation. Slurry dehydration that occurs and proppant density throughout the fracture area are thereby calculated4,5. The TerraFRAC™ simulator used here allows calculation of the fracture (or inflation of the fracture) as pumping continues, and does not require an arbitrary definition of *ldquo;TSO" and a termination of the calculation. Proppant density at each time step in the calculation and at all locations throughout the fracture may be viewed, and screenout - i.e. the reaching of a critical proppant density where further packing is not physically possible - is always known. This is very important, as screenout may therefore be carefully studied including the onset, the transition to complete tip screenout, the inflation of the fracture, and finally further extension or breakout of the fracture. This helps to determine net pressure allowable before fracture breakout and to avoid overestimates of fracture width. Case Descriptions Several cases have been investigated to understandthe effect of the perforation interval (relative to the total pay zone) on fracture containment andthe ability to avoid fracture growth downward into what might be a water saturated zone. In all cases TSO was obtained, and net pressure is noted along with fracture geometry, fracture width, and proppant concentration throughout the fracture.