Abstract

While hydraulic fracturing is the most commonly used stimulation method for the Codell tight sand at the Wattenberg Field, the authors have observed that the leak-off tests during a typical frac can be utilized to calculate the near wellbore reservoir pressure, far-field reservoir pressure, and permeability indicator. The correlations calculating the reservoir pressures are based on the linear relationship between the fracture closure pressure and reservoir pressure, whereas the permeability indicator is inferred from an analysis of the leak-off coefficiency, or a critical factor of the formation transmissibility. The validity of these methods has been substantiated by the corresponding pertrophysical property distribution, as well as well performances. As the results of these methods, the distribution of the calculated reservoir pressures and permeability indicator have greatly benefited the infill drilling program in terms of situating new in high pressure and high permeability areas while avoiding depleted places.

Introduction

The Wattenberg Field is located in the western part of the Denver Basin, Colorado (Figure 1). It was discovered and began producing in 1970. The Codell Sandstone is the primary reservoir. It comprises tight, marine-shelf sandstone deposited under low energy conditions. The sandstone is silty, argillaceous, bioturbated, and very fine-grained with abundant clay. Averaged core porosity is 10–12%. Permeability is below 0.5 mD. The initial porosity and permeability are low because of the high clay matrix, and have been further reduced by compaction and diagenesis. The original reservoir pressure gradient is about 0.6 psi/feet. After a prolonged development, depletion is evident in many areas, as a result of combined effects of infill drilling program and heterogeneinity in reservoir properties. Because of its tight nature of the Codell Sandstone, all the wells have to be stimulated by hydraulic fracturing (Weimer et al., 1986; Boone et al., 1998).

A typical frac job in the Codell Sandstone starts with a leak-off test, which lasts for about five minutes. During this test, all pumps are shut down, and the pressure declining curve is recorded against time. As of the end of 2007, more than one thousand leak-off tests had been collected from the fracs taking place in the Codell Sandstone. In conjunction with the reservoir pressure tests performed in the Codell formation, the authors have examined the pressure declining characteristics of the leak-off tests and developed correlations to estimate the reservoir pressures and a permeability indicator using the leak-off test information.

The development of the methods to calculate the near-wellbore reservoir pressure and far-field reservoir pressure

In order to initiate the infill drilling in the Codell Sandstone, Aanakarko Petroleum started a project to measure the reservoir pressures and verify the depletion condition in 2005. The Diagnostic Fracturing Injection Tests (DFIT) have been performed in more than 30 wells where the Codell reservoir pressure, fracture closing pressure, permeability and pressure dependent leak-off have been obtained.

In the same wells where the DFIT was conducted, the authors examined the pressure declining profiles of the leak-off tests carried out at the beginning of the frac jobs. Figure 2 demonstrates the surface pressure curve recorded from a typical frac, where the 5-minute leak-off test is highlighted. After zooming in (Figure 3), it is noticed that the later portion of the pressure declining is more stabilized and noisy free. From each leak-off test, the Initial Shut-In Pressure (ISIP) has been picked at the end of the test. For the sake of consistency, all the ISIP was picked at the end of the fifth minute.

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