WELL TESTING COURSE. Authors: Enzo Beretta ENI E&P. Francesca Verga Politecnico di Torino

Transcription

1 2004 WELL TESTING COURSE Authors: Enzo Beretta ENI E&P Francesca Verga Politecnico di Torino

2 Preface The material gathered in this volume was first presented and discussed during the Well Testing course held in 1998, in S. Donato (MI) and was further reviewed for the subsequent editions of the Well Testing course. These slides are aimed at providing with some reference guidelines both those engineers and geologists who are already familiar with well test analysis and those who are just approaching the subject. It is not meant to be a handbook and, therefore, the authors recommend that the readers refer to textbooks or to appropriate research papers available from the literature for a deeper understanding of the basic theory and current practice of well testing. The mathematical theory behind the presented equations and models is not explained into details, only the underlying assumptions are summarized. Note that the coefficients appearing in the formulae are coherent with the S.I. system of units, so readers who use other systems of units should adapt the formulae to their particular systems. SECTION 1 provides an overview on well testing targets and standards, equipment and gauge specifications, field data collection and quality control. SECTION 2 deals with flow equations for both oil and gas reservoirs. Reservoirs are first regarded as infinite acting systems; then bounded reservoirs are taken into account. The main well testing operating procedures, the flow equations used to interpret the field data, and the parameters that can be obtained are outlined in the case of both oil and gas wells. Eventually, an introduction to the pressure derivative method, currently used worldwide for well test interpretation, is presented. SECTION 3 thoroughly describes all the theoretical reservoir models together with the respective pressure derivative responses that have been developed in the literature and are available in any software for well test analysis. The models are classified as early time models to evaluate wellbore and near wellbore effects, middle time models to investigate the reservoir behavior, and late time models to infer possible boundary effects.

3 Well Testing The Principle of Well Testing is... to analyse the output signal of a well on which a known input signal has been applied. INPUT well OUTPUT Production / Injection flow rates Bottom hole pressure

4 Standard Well Test RATE and BHP -vs- time 3000 BHP, psia RATE TIME, hours

5 Main Targets of Well Test define nature of produced fluids and well deliverability estimate reservoir properties ( i.e : Pf, K, Skin ) evaluate reservoir size ( drenage area ) analyse reservoir heterogeneities ( i.e : sealing boundaries ) design remedial jobs ( i.e : acid treatment / gravel packs / fracturing ) verify completion efficiency design the surface production facilities and for new infilling wells : optimise drilling technique by choosing the best mud in order to minimise formation damage

6 Types of Well Test DST Standard Production Test Interference Test (areal /vertical) Limit Test

7 Well Test Type DST Well : exploration Completion : DST string Duration : very short ( few hours ) Objectives : 1) Fluid samples 2) Formation pressure

8 Standard P.T. Well Test Type Well : exploration / producing Completion : final Duration : variable ( 2-7 days ) Objectives : 1) Pf, K, well damage 2) Reservoir boundary 3) Well deliverability

9 Areal Interference Well Test Type First Well : active ( producing ) well Second Well : observer well ( shut - in ) Duration : variable ( days - weeks ) Objective : Recognise reservoir continuity between wells

10 Vertical Interference Well Test Type First Pool : active ( producing ) pool Second Pool : observer pool ( shut - in ) Duration : variable ( hours - days ) Objective : Recognise within the well pools comunication

11 Well Test Type Limit Test Well : exploration / producing Completion : final Duration : variable ( days - weeks ) Objectives : 1) Investigate reservoir boundaries 2) Estimate reservoir size

12 Well Testing: Field Data Field Data Surface Data Bottom Data Produced Fluids Wellhead Pressure Wellhead Temperature Pressure Temperature Flow Rates Field Fluid Properties DWT SRO Memory Gauge Separators

13 Well Testing: Field Data Quality Control Bottom Pressure / Temperature : Always check the coherence of the instrumental response in terms of both pressure and temperature ( Validation Gauges - INT/2 ). Always check bottomhole pressures against the corresponding wellhead pressures (DWT) taken as a reference. Flow rates : The flow rates selected in the Rate History must be homogeneous with the corresponding bottomhole p ( Validation Rates - INT/2 ).

14 Well Testing Equipment Surface equipment : a) Bi-threephase separators b) Choke manifold c) Dead weight tester ( DWT ) Downhole gauges: a) mechanical ( Amerada ) b) electronic : Memory gauge SRO gauge

15 Downhole gauge specifications Mechanical Gauge ( Amerada ) Max W.P. : up to 20 Kpsia Max W.T. : up to 200 C Resolution : 0.05% full scale ( 5 10Kpsi ) Accuracy : 0.4 % full scale ( 40 10Kpsi ) Drift : 10 psi/1 day - 10 psi/week NOWADAYS HARDLY EVER USED

16 Downhole gauge specifications Electronic Gauges ( Memory / SRO ) Max W.P. : 20 Kpsia - Max W.T. : C Strain Gauge Quartz Gauge Resolution : 0.2 psia 0.01 psia Accuracy : 10 psia 2 psia Drift : 3 psi/1 day negligible or 1.5psi/week

17 Gauges specifications

18 Microsystems-Technology Courtesy of Schlumberger

19 Well Testing: Simulation Phase-INT/2 Starting Analysis : Input Phase Geometrical Petrophysical PVT Rate History BHP Q.C. Q.C. Simulation