Abstract

This paper presents a new quartz electronic memory gauge that has been introduced to the oil industry. This gauge can record data with more precision than existing gauges, and thus, can establish more accurate flowing and shut-in pressure with higher resolution. In addition to describing the gauge design, this paper will also discuss the advantages of improved gauge resolution in the analysis of well test data and in determination of the hydrocarbons in place. The impact of using a higher resolution gauge on well testing is illustrated through analysis of field data.

Introduction

The proper selection of a pressure-measuring device is a critical part of well testing. Many factors should be considered before selecting the appropriate gauge for a test. Gauge resolution should be considered the most important factor for conducting a short-term test. Other important factors include reliability, accuracy, drift characteristics and cost.

A well-test engineer must have high quality data in order to perform an accurate pressure transient analysis. Often, with traditional gauges, the last flowing pressure and time are not recorded correctly. This deficiency can be attributed to insufficient pressure data around shut-in time (or any rate changes), a pressure gauge with limited accuracy and resolution, or from closing the tester valve too slowly. Hardware1,4 or software1–10 solutions for such a problem are highly desirable and have been presented in the literature. This paper concentrates on producing a new hardware solution to the problem.

Producing or injecting at very high flow rates, especially in low-permeability and high-pressure formations, may cause significant pressure changes at the instant the well is shut-in for a pressure buildup. The presence of skin damage will amplify this sudden pressure change. All of these nonideal closure situations will make the wellbore storage effects deviate from the unit-slope line on the log-log plot. This may confuse the engineer so that the storage period is not properly identified, and the data may be incorrectly assessed as results of fracture flow, a changing-wellbore storage, a high formation storage close to the wellbore, or as a phase-segregation effect. In some cases, well testing engineers can assume these are early-time anomalies and totally ignore them without attempting to find and correct the source of the problem. Typically, this may lead to lower skin and permeability assessment. Numerous cases have been encountered where the engineer calculated skin values, which were not high enough to do any type of formation stimulation, but the production/injection of these wells had indicated lower-than-historical productivity or injectivity. After the early-time data of such wells had been corrected, much higher skin and permeability values were obtained.

This paper will provide a detailed description of a newly-designed, fast-recording, high-accuracy quartz electronic memory gauge that provides a significant improvement in pressure resolution when compared to the quartz gauges presently on the market. Since this gauge can record data more precisely, pwf can be established more accurately. This improvement in resolution is the result of a sophisticated proprietary algorithm for collecting memory gauge data and an innovative low-power automatic gain control for driving the quartz crystal.

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