Practical Application of Calculated Multiphase Flowing Bottom-Hole Pressures
Authors: Kern C.P. , Nicholson F.R. 
Published: December 2007

Flowing bottom-hole pressures accurately calculated from surface measurements can define both well and reservoir parameters from routine well tests. Heretofore, the practicality of this has been questionable due to the large number of well tests and the time required for each calculation. With the advent of high speed digital computers to make these calculations, this method has become most useful. Producing rate, reservoir pressure and fluid properties in conjunction with flowing bottom-hole pressure form the basis for calculating productivity index, effective permeability and well capacity. Calculated effective permeability combined with future well capacity has been used to improve sweep model studies. A comparison is included of a sweep model study using these principles, as opposed to using constant well capacity and average permeability. The effects of chokes on surface flowing pressures can be predicted and several examples are included. A technique to predict gas lift performance is given. When the reservoir producing mechanism has been determined, a prediction on an individual well basis can now be made.

Introduction

During the 1930's, with the work of Uren and Moore et al., an investigation into the principles and factors involved in defining multiphase flow in vertical conduits was begun. This work is not yet finished. However, today many correlations are found in the literature which make calculated flowing bottom-hole pressures sufficiently accurate to have practical applications. Use of high speed digital computers enables the engineer to analyze the results of past test data in a new light. Of the many contemporary methods of calculating flowing bottom-hole pressures, the correlations of Ros more nearly approximate the measured pressures in the fields under consideration. In this paper, the Ros method has been used exclusively. Presented are applications of productivity indices and effective permeability derived from calculated flowing bottom-hole pressures. Reliable individual well and reservoir performance predictions can now be made.

Calculation of Flowing Bottom-Hole Pressure

A comparison between a flowing bottom-hole pressure calculated by a computer program based upon the Ros method compared to a field-measured flowing bottom-hole pressure is included in Table 1. The first three lines of the table serve as identification and general information while the succeeding five lines (tubing pressure, oil, gas and water production rates with gas-oil ratio) form the field-measured basic input data for the calculation. This routine information is normally maintained as a matter of company records or is publicly available from the various state regulatory bodies. In addition to the test information, it is necessary to have knowledge of pressure-volume-temperature relationships of the hydrocarbon system, the depth to which the calculation is to be made, conduit size and whether annular or tubing flow. Shown next is the calculated flowing bottom-hole pressure which differs by 55 psi from the measured pressure. Usually, flowing bottom-hole pressures can be calculated to an accuracy within 5 to 10 per cent of the pressure drop in the flow conduit. This discrepancy will produce usable data in a reservoir which demonstrates a large wellbore drawdown.

JPT

P. 1373