The paper was presented at the 59th Annual Technical Conference and Exhibition held in Houston, Texas, September 16–19, 1984. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write SPE, 6200 North Central Expressway, Drawer 64706, Dallas, Texas 75206 USA. Telex 73989 SPEDAL.

Abstract

The risk and cost involved in undertaking bottom-hole pressure surveys by wireline in high-pressure gas wells, especially those which produce sour gas, often discourages obtaining produce sour gas, often discourages obtaining information which is vital for evaluation of well performance. performance. Field tests have verified the accuracy of a sensitive, high-pressure acoustic instrument that enables determination of fluid levels and/or echo-time to a downhole marker, without limitation by depth, well tubulars or environment. This has opened the way for development of a diagnostic technique giving information about well pressures and fluid distribution in the wellbore without the use of downhole pressure survey tools. This Wellsite Pressure Calculation System (WPCS) is especially applicable for hostile environments, attributable to high pressure/high temperature, sour gas production, where wireline surveys involve significant risk and expense.

Results, supported by field tests involving pressure buildups under controlled conditions, pressure buildups under controlled conditions, indicate the calculated pressures agree with those measured by downhole pressure bombs to within the accuracy of the pressure bombs (1%) and can be used satisfactorily in buildup analysis.

A description of three sets of field tests is given, as well as the development of programs for wellsite data analysis using a portable, battery-powdered microcomputer.

Introduction

Evaluation of well performance requires knowledge of bottomhole pressure at relatively frequent time intervals during the producing life of a well. Also, conditions requiring workovers, stimulation, inhibition of corrosion and/or paraffin deposition can be evaluated with more paraffin deposition can be evaluated with more precision if the distribution of fluids in the precision if the distribution of fluids in the wellbore can be known with reasonable confidence. Direct measurement, via wireline-conveyed pressure bomb, of pressure as a function of depth and the subsequent calculation of pressure gradients, plotting of pressure traverses and pressure build-ups, plotting of pressure traverses and pressure build-ups, are the conventional methods for evaluation of well characteristics.

The cost, complexity and potential risks associated with wireline methods applied to high pressure wells in general and sour wells in particular, pressure wells in general and sour wells in particular, have created the incentive to develop pressure survey techniques based on calculations from surface measurements of pressure and/or liquid level in the wellbore. Such techniques are extensions of applications of acoustical well sounders in analyzing the performance of normal pressure oil producers. In oil wells the pressure oil producers. In oil wells the acoustic surveys are performed in the tubing-casing annulus where the reflection from the gas/liquid interface can be correlated with reflections from the tubing collars thus providing a depth calibration. Application of the technique to gas wells or to wells with a sealed annulus implies undertaking the survey through the tubing. This resulted in reflection records of limited quality where interpretation could not be undertaken with confidence. Andsager and Knapp proposed using calculated sound velocity in the gas as a means for interpretation and calculation of the position of the liquid level. Field application of their technique to 38 surveys resulted in determination of the liquid level to within 2% of the actual location for 75% of the measurements and with a maximum error of 4.2%. Further refinement was reported by Thomas et al. by introducing a more rigorous calculation of sound velocity. This reduced the average error for calculation of the distance to 0.71%. These applications involved relatively low pressure gas wells ranging from 935 to 2203 psig wellhead pressures and depths of approximately 7000 feet.

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