A New Bottom-Hole Pressure Gauge
Authors: Jones Jack W. , Bennett John D. 
Published: December 2007

The Sun Precision bottom-hole pressure gauge is 9 ft. in length and a maximum of 1 1/2 in. in diameter. It is completely self-contained and self-recording. If has a sensitivity of approximately 0.005 per cent of full scale rating and a reproducible accuracy of +/–0.025 per cent of full scale. If can be used on short-term tests such as static field surveys or gradient studies. If can also be used on long-term tests such as drawdown, build-up or interference tests with durations up to 28 days.

Introduction

Pressure is probably the most important directly measurable parameter of an oil reservoir. From pressure many of the important variables can be derived which can be used in evaluating a well or reservoir, method of production, effectiveness of secondary recovery processes, etc. Reservoir engineers can determine effective permeability, porosity, effective drainage radius, extent of wellbore damage if such exists, and the continuity of the aquifer, oil zone or gas cap from various pressure measurements. These include transient pressure measurements under drawdown or build-up conditions and interference effects in a shut-in well due to a change in production or injection in another well. The importance of pressure in the determination of the above mentioned parameters was the motive for initiating a program for the development of a precision down-hole pressure gauge. After reviewing pressure transducers, it was decided to use the bourdon tube as the basic pressure sensor. Our design was directed at achieving the accuracy and precision goals by a precision method of detecting the angular rotation of the bourdon-tube shaft and an increase in resolution of the recording systems.

Description Of The Gauge

Fig. 1 is a functional block diagram of the pressure gauge. Basically the gauge can be divided into two functional units, which are the pressure measuring system and the recording system.

Pressure Measuring System This system includes the bourdon tube, sensing mechanism, high and low pressure contacts, gear-reduction nulling unit, motor, clock and electronics control circuit. The bourdon tube is the basic pressure transducer. Its output shaft rotates through an angle proportional to the pressure applied to it. The sensing mechanism is a differential transformer, the secondary coil of which is connected to the output shaft of the bourdon tube and rotates with it. It is mounted in jewel bearings which provide practically frictionless load on the angular movements of the bourdon-tube shaft. The primary coil is connected to the gear-reduction nulling unit. During a pressure measurement the primary coil is rotated by the motor and gear- reduction nulling unit, under control of the electronics control circuit, until it is aligned in a specific angular position with respect to the secondary coil. This angular position is the null position of the two coils. The system has a precision and accuracy of restoring the primary coil to this null position of =/–0.005 degrees.

The gauge does not continuously measure pressure, but samples the pressure either on a time basis actuated by the clock or according to increasing or decreasing pressures, in which case the high or low pressure contacts actuate the measurements.

JPT

P. 1277^