New Early Formation Pressure System Field Test Results and Advances in Early Time Pressure Buildup Analysis
Authors: Skinner Neal G. , Proett Mark A. , Ringgenberg Paul D. , Chin Wilson C. , Aadireddy Reddy 
Published: December 2007

This paper presents field test results of a recently introduced Early Formation Pressure System (EFPS). The EFPS can be used in a wiper trip during a drilling program to perform a series of quick formation pressure tests similar to those run with a wireline formation tester. The EFPS consists of two inflatable packers, a pump, and high precision quartz gauges and is conveyed on drill pipe. Drill pipe pressure is used to inflate the packers, and pipe reciprocation is used to initiate drawdowns. Typically, the tool is deployed during wiper trips and the bottom hole assembly can include any type of logging-while-drilling (LWD) tools.

The field test results of the EFPS are analyzed with a new exact solution of the spherical flow well test equation that is valid for all time. The solution is used to predict formation pressure and permeability from early to intermediate time pressure transients. This scope is not possible with conventional drill stem test analytical models because they do not model the entire time behavior of the pressure response. The new exact spherical flow model, derived from first principles, includes general wellbore storage effects and is solved in closed, analytical form, thus permitting convenient pressure response to theory matching using the complete time regime (early, transitional, and late time data). Well test examples demonstrate accurate pore pressure and permeability predictions from the EFPS, and detailed numerical simulations over a wide range of conditions illustrate the utility and power of the new technique.

Introduction

Historically, two methods have been used to evaluate downhole formation pressures after drilling a well prior to completion. The oldest method is the drill stem test or DST which dates back to the 1930's. A DST is a temporary completion of an openhole or cased hole well through the drill pipe to gather reservoir data. Drill stem tests can have a tremendous range of complexity. A 2,000-ft test on land is much simpler to plan and execute than a high pressure, high temperature DST from an offshore floater; however, there are many similarities. In each case, a set of DST tools is run into the well on drill pipe or tubing. A packer isolates a zone of interest from the hydrostatic head of the column of drilling or completion fluid above the formation. A tester valve opens and closes to flow and shut in the well. Pressure gauges record the pressure history during the drawdowns that occur when the well flows and during the buildups, when the well is shut in. At the conclusion of the DST, a circulating valve opens; the well is circulated, and the DST tools and the pipe are withdrawn from the hole.

The wireline formation tester (or WFT) was introduced in the mid 1950's as a sampling tool, and has grown in acceptance as an openhole service for pressure testing and sampling during the past 20 years. A WFT runs in the hole on electric wireline. Instead of sealing a section of formation with a packer, a rubber donut-shaped pad is pressed against the side of the wellbore to isolate a small zone of interest from mud hydrostatic. Formation fluid is drawn through the pad into the WFT. This is usually done by moving an adjustable piston inside the tool. As the fluid is drawn into the tool, formation pressure drops. After the fluid has been drawn from the formation, formation pressure will build up. During the test, real-time formation pressure measurements are transmitted to the surface via the electric wireline At the conclusion of the test, the pad is released from the wellbore and the WFT is repositioned to test at another location in the well.

Recently, a new generation of WFTs was introduced. In this new class of enhanced WFTs, an electric pump moves fluid from the formation during the test. These newer tools have increased WFT flow rates and volumes and are more versatile than the earlier WFT tools.