Pressure Transient Expert Analysis

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What is Pressure Transient Testing

Pressure Transient Testing commonly referred to as ‘Well Testing’ is a routine procedure in the oil industry aimed at ascertaining how good a well can produce or inject. Well Testing also helps determine reservoir features close to the well (near wellbore) as well as those at a given distance from the well. Well testing involves subjecting the well to series of rate changes (including shut-ins) whilst measuring the pressure transient response. Well testing is usually designed to incorporate factors such as type of hydrocarbon fluid, reservoir type, resolution of the measuring device and the overall test objective.

What is Pressure Transient Testing

Why should a well be tested?

A Well is usually tested to determine factors like flow capacity, fluid type, initial reservoir pressure, reservoir heterogeneity, possible drive mechanisms, boundaries & compartmentalisation, lateral and vertical hydraulic communication, geological structure and overall well condition.

Types of Pressure Transient Testing

Various well pressure transient testing techniques are employed based on the stage of field life. Some of the common types include:

  • Standard and Mini Drill Stem Testing (DSTs)
  • Production, Injection and Interference Tests
  • Permanent Down Hole Gauges (PDHGs)
  • Wireline Formations Testers
  • Time Lapse Well Testing

Drill Stem Tests (DSTs)

DSTs and mini DSTs are mainly employed at the exploration stage of the field life. It confirms exploration hypotheses and provides basis for initial production forecast. They can be used for more detailed reservoir characterisation and well performance purposes at the appraisal stage of a field. DSTs and Mini DSTs can also be used for injection well test in depleted formations or tight formations; mini-fracture testing in tight reservoirs.

Drill Stem Tests

Permanent Downhole Gauges

Permanent Downhole Gauges (PDHGs) continuously monitor well and reservoir pressure changes. In the past this was mainly used to assess and predict reservoir performance. Due to large volume of data associated with PDHGs, a special analysis technique, known as Deconvolution has provided an effective way of optimising the data while deriving useful information for well, reservoir and field management.pta7 In development wells, production testing involves surface control and the need to understand the effect of storage on pressure transient analysis is equally important. Pressure transient analysis using permanent downhole gauges can provide useful information on potential problems in the well production systems and appropriate solutions to mitigate these well problems. The effectiveness of remedial and workover operations carried out on a well can also be got from pressure transient testing.

Permanent Downhole Gauges

Formation Testing

With increasing need for environment considerations, formation testers have become increasingly crucial to reservoir evaluation in petroleum exploration. Formation testers provide alternatives to conventional well testing in exploration and appraisal wells considering low cost associated with deploying the tools and also reduction in revenue loss during build-ups.

Our in-house experts can provide operational support during data acquisition for the various wireline formation tests in different environments.

The choice of type of Well Formation Tester (WFT) testing and tool section would depend on test objectives; reservoir and well conditions; cost and other environment consideration. In instances where pre-test is not applicable, probe pump out or mini DST maybe best.

Considering the small volume of reservoir fluid that are displaced in WFT compared to conventional well testing and interaction of tools with measured well response, specialised methods which are different from conventional well testing are required in using WFT data for pressure transient analysis.

Wireline formation testing (WFT) maybe confronted with problems such as insufficient gauge resolution to record interpretable pressure data in high permeability reservoirs; supercharging during pressure measurement; and noise. These challenges can be overcome by proper job design and application of interpretation techniques to obtain useful information for reservoir characterisation; completion strategy; optimization of mud for infill drilling; detection of fluid contact movement.

Understanding reservoir layering and characterising lateral continuity of barriers is very important in reservoir development. WFT provides useful information about vertical communication across the reservoir.

Formation

Deconvolution

Deconvolution has changed the way tests are designed considering it can be applicable to various pressure transient tests including DSTs, permanent downhole gauges; analysis of slug test and closed chamber testing. Ability of deconvolution to analyse well test data at variable rate has made it important in analysing large set of well test data from permanent downhole gauges ranging from long months to several years. Such data contain information about the reservoir at distances several orders of magnitude larger than the radius of investigation from of single flow period. Deconvolution enables automatic correction of rate errors making it a useful tool for estimating correct rates during well testing especially for permanent bottom hole gauges which are often characterised by significant rate error and slug test where no rate is measured except commutative liquid volume from loading.

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Time-Lapse Well Testing

In situations where changes in reservoir occurs due to changing water saturation distribution; formation of condensate or gas bank, time-lapse well test-pressure derivatives can be used to monitor changing fluid saturations in these reservoirs. They can further provide useful information relating to water movement; increasing condensate bank in gas condensate reservoirs and gas banks in volatile oil reservoir.

Designing a Pressure Transient

It is very important to forecast possible well test response in exploration, appraisal and development wells within practical limits in order to increase the probability that desired well test objectives are met during actual pressure testing. In order to forecast possible response that would satisfy test objectives, forward simulation of pressure test using range of parameters that are considered reasonable is very important. In most test design for pressure transient application purposes, test objectives can be to determine when specific flow regimes manifest; determine best rate sequence for well test; refine existing test etc. For example, in development wells, well fracture design is important to understand what potential benefit maybe derived from such well stimulation process prior to actual operations. Proper test designs saves money and time ensuring test objectives are met and optimised.

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