Overpressure Calibrated Design of Hydraulic Fracture Stimulations

Abstract

ABSTRACT This paper presents a new concept, model, and procedure to design hydraulic fracturing treatments. The purpose of the paper is to develop an approach in designing hydraulic fracture stimulations that would be more consistent with field observations and would better match the field net fracture propagation pressure. The concept and procedure consist of using the net fracture pressure measured shortly after shut-in (overpressure) to determine the apparent fracture toughness (Γ or K1c) of the formation. This field calibrated K1c or overpressure is then used as an input to an approximate hydraulic fracture simulator, called ENERFRAC, to design the hydraulic fracture stimulation volume and proppant injection schedule. The field overpressure needed to determine the K1c can be accurately measured using a minifrac test. The overpressure is defined as the difference between the corrected instantaneous shut-in pressure and the minimum in-situ stress. Frequently, the field calibrated K1c could be one to two orders of magnitude larger than those measured in the laboratory using small samples. This high value of K1c is attributed to the field and scale dependent process zone at the fracture tip. The field calibrated K1c could dominate the fracture growth and needs to be included in the hydraulic fracture model to predict realistic fracture dimensions. This paper provides an approximate model, called ENERFRAC, that incorporates the effect of K1c in addition to the other interacting processes of viscous fluid flow, elastic rock deformation, and fluid loss in the conventional models of contained and uncontained fractures. This paper outlines the procedure for the overpressure calibrated design and provides three field examples to illustrate the design procedure and to show the effect of designing with and without overpressures. Examples of treatments that are more aggressive (with smaller pad and larger amount of proppant) or more conservative (with larger pad) than those generated by the Khristianovic/Geertsma/deKlerk model (KGD) are presented. Thus, the proposed overpressure calibrated design not only provides a stimulation that is more consistent with field observations but may also reduce the risk of premature sand-out.