Abstract

Using a computer solution of the equilibrium fracture height model, fracture height growth out of the treated zone simulated by comparison of the closure stress of the upper and lower boundaries with the closure stress. of the pay zone.

Results from this model are compared to actual achieved fracture height measured from after-frac survey. on a diverse group of oil and gas wells. Pressure data are analyzed on a selected number of Pressure data are analyzed on a selected number of wells to demonstrate the effect of varying pore pressure on the model. pressure on the model. Utilizing this data, correlations between log-derived rock stress and fracture height growth are developed which will allow accurate prediction of total fracture height and containment pressures.

Introduction

Recent technological advances in the use of digitized long-spaced acoustic data now permit accurate measurement of shear and compressional interval transit times allowing the determination of basic rock mechanical property data such as Young's Modulus, Poisson's Ratio, Shear Modulus and Bulk Compressibility. Using these fundamental rock properties together with other log properties as properties together with other log properties as porosity and bulk density, it is possible to model porosity and bulk density, it is possible to model a dynamic (in situ) closure stress.

Based on earlier work by Simson, Rosepiler Warpinski and Settari, in 1985 Newberry et al. proposed that fracture height growth was the result proposed that fracture height growth was the result of achieving downhole treating pressures in excess of the pressure needed for crack extension. Their model further suggested that growth of the fracture could be simulated by comparison of the closure stress of the upper and lower boundaries with the closure stress as of the zone being treated. Later, Ahmed et al. determined a method for solving height growth for multiple zone completions using an iterative application of the Newberry et al. model.

Using these techniques to improve stimulation effectiveness, actual height growth was monitored through use of after-frac surveys. Common surveys that were run included temperature and radioactive tracer surveys.

By comparison of achieved fracture height with modeled fracture height in wellbores having a variety of formation conditions (depth, lithology, temperature, pressure, etc.) the validity of this model was tested and verified. Utilizing these data, this research provides a verified correlation between log-derived rock properties and downhole fracture propagation.

Background

Over the years, drillers of oil and gas wells looked for improved methods to increase production. Many different techniques were tried with varying degrees of success until the onset of hydraulic fracturing in 1947 in Kansas. This first documented instance of pressuring the wellbore proved to be a safe add pressuring the wellbore proved to be a safe add efficient means of increasing hydrocarbon yields. Since then, tens of thousands of wells world-wide have been hydraulically stimulated. This procedure is now used on nearly 40% of all wells. While many of these treated wells are economic successes, some are not. in order for the economics to be justified, the stimulation process must first be a technical success.

A fracture induced in a formation always follows the path of least resistance, normal to the least path of least resistance, normal to the least principal stress. There are two horizontal principal principal stress. There are two horizontal principal stresses, Shmin and Shmax and the vertical or overburden principal stress SV.

In most areas which are tectonically inactive, SV> Shim, therefore the fracture propagate. in a vertical plane, normal to the direction of Shmin in some areas of tectonic compression and at shallow depths, or where a large overburden has been removed over geologic time, it is possible that horizontal propagation will take place. The aspect of propagation will take place. The aspect of horizontal fracturing is lot a common situation in the petroleum industry and not within the scope of this work, although all the basic tenets of rock mechanics discussed here still apply.

Drilling a borehole and filling it with a fluid distorts the horizontal stress field near the wellbore, leaving the overburden unchanged. At the borehole wall, the horizontal principal stresses are radial and tangential.

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