This paper presents a practical method to estimate initial reservoir pressure from early flowback data in fracture-stimulated horizontal wells. It is based on estimating flowing bottomhole pressure (pwf) near the time of first hydrocarbon production, while the well is flowing back frac load. The method is important because of the difficulty in measuring initial reservoir pressure in extremely low-permeability unconventional reservoirs. It requires no special instrumentation, and no modifications to flowback procedures (as long as pwf reduction is gradual). Pressures and rates measured hourly at the surface, along with fluid properties and wellbore configuration, are all that is needed.
Field examples show the hourly pwf behavior from the beginning of flowback (100% water) to the appearance of first oil or gas, through increasing oil and gas rates as cleanup progresses. Calculated pwf starts out above reservoir pressure due to ‘frac charge’, falls until it levels out or increases near the time of first hydrocarbon production, then declines. The flattening of calculated pwf prior to first measurable hydrocarbon production provides an estimate of initial reservoir pressure. Examples are from horizontal oil and gas wells in the Woodford and Meramec formations of the Anadarko Basin in Oklahoma.
The accuracy of calculating pwf from surface data is high while the well is flowing 100% water; this period is relied on to estimate reservoir pressure. Once measurable hydrocarbons are reported, multiphase flow correlations are used. Estimated values of pwf from correlations are compared with measured values from flowing gradient surveys, showing acceptable error for both oil and gas wells in the subject area.
The applicability of other techniques for estimating initial reservoir pressure in unconventional reservoirs is discussed. The importance of accurate initial reservoir pressure and pwf over time are discussed, along with the use of such data in play assessment and development. An example shows how trends in reservoir pressure over a large Woodford play were identified by applying the technique on every well.
The method and examples described herein will enable engineers to recognize typical profiles of wellhead pressure and pwf during flowback, and to estimate initial reservoir pressure in wells where direct measurement is difficult or impossible.