Rock Mechanics Laboratory Tests for Petroleum Applications. Rob Marsden Reservoir Geomechanics Advisor Gatwick

Transcription

1 Rock Mechanics Laboratory Tests for Petroleum Applications Rob Marsden Reservoir Geomechanics Advisor Gatwick

2 Summary A wide range of well established and proven laboratory tests are available for petroleum rock mechanics studies. Not only can these provide fundamental rock properties for characterization purposes or for well designs, the more specialist tests can provide information on how formations might behave when they are subjected to complex stress-paths around wellbores or within reservoirs at various stages during the field-life. Laboratory studies also provide methods for obtaining data that can not otherwise be derived from log data or history matching. For example, whilst strength properties can only be inferred empirically from log data, they may be determined directly from laboratory measurements. Similarly, whilst logs can provide data on dynamic elastic properties, their static (i.e., mechanical) elastic and non-elastic deformation parameters can only be derived from history matching or laboratory tests. Continuous log data combined with laboratory tests on selected samples can therefore provide complementary tools for use in petroleum geomechanics. When coupled with petrophysical studies, the simulation of realistic mechanical loading on a laboratory sample can also significantly enhance the reliability and usefulness of what might otherwise be routine petrophysical analyses. Laboratory techniques also exist for obtaining in situ stress data and, since these methods are based on fundamentally different processes to those employed in hydraulic fracturing etc., they provide useful and complementary adjuncts to rig-site methods. This presentation describes the type of rock mechanics tests available, including some of the more specialist studies as well as routine measurements. The types of equipment employed are introduced, and the data obtained from such laboratory programmes are explained in the context of their end-use for well engineering and reservoir engineering applications.

3 Rock Mechanics Tests.. routinely involve measurements of: Strength & deformation characteristics Pore pressure responses under un-drained loading Porosity and bulk volume changes Fluid transport characteristics Influences of temperature, effective stress path etc. and measurements of: Ultrasonic velocities AE

4 Essential Capabilities: Simulate deviatoric stresses at depth or in proximity to a wellbore Measure deformations and bulk/pore volume changes Systems typically incorporate : Triaxial or confining cell Load or reaction frame Closed-loop loading, confining & pore pressure systems Heating systems (for HT applications) Devices for measuring deformations, pore volume changes, flow, ultrasonic velocities

5 Closed-Loop Control i.e., programmed rate of displacement f p / t is selected actual displacement f a is measured difference f p -f a feeds back to the system control system responds, correcting f p -f a to zero Fluid reservoir Confining pressure controller Program Hydraulic power f p Logger Amp f a -f p f a Correcting signal Cell pressure Piston displacement monitored Pressure intensifier t Triaxial frame & cell Hydraulic power Correcting signal Servo-valve Load Amp f a -f p f 1 f 2 Axial displacement Axial load controller Program f p t Select feed-back Logger

6 Examples of Advanced Test Systems 2000kN capacity system (configured for triaxial tests) 1600kN capacity dynamic system 3000kN capacity HPHT system

7 Uniaxial Compression Testing Axial stress (MPa) Mean diametral strain (mstr) Indirect methods: Schmidt hammer Brinell hardness Penetrometer Point load Axial strain (mstr) Axial stress Mean diametral strain Static Young s modulus σ 3 = 0 Static Poisson s ratio σ 3 = 0 Uniaxial yield strength (σ Y ) Uniaxial compressive strength (UCS)

8 Triaxial Testing Conventional triaxial compression: Lateral stresses are σ 2 =σ 3 Axial stress σ 1 is increased Triaxial extension: Lateral stresses are σ 1 =σ 2 Axial stress σ 3 is reduced Data obtained: Triaxial yield strength (σ Y ) Peak strength Residual strength (σ R ) Static Young s modulus (E) at confinement Static Poisson s ratio (v) at confinement Configuration for coupled HPHT

9 Triaxial Deformation & Peak Strength Data Axial deviator stress (σ 1 -σ 3 ) Increasing confinement Major effective stress σ 1 Impossible state Possible states Shear stress Curved failure envelope Individual test data Normal stress Axial strain Uniaxial compressive strength UCS Tensile strength Minor effective stress σ 3

10 Multi-stage triaxial compression (or extension) Confining stress, MPa Axial stress, MPa Axial strain, millistrain Diametral strain, millistrain e.g., σ 3 = MP Axial strain, millistrain Axial stress Diametral strain 1 Diametral strain 2

11 Multiple Measurements on Single Plug K Bulk E, v, C, & φ σ 3 σ 1 Axial stress (MPa) Axial strain (mstr) Mean diametral strain (mstr) K Grain σ 1 =12 MPa σ 3 =12 MPa u =10 MPa 5-stage σ 3 =22, 25, 30, 35, 40 MPa u =10 MPa Axial stress Mean diametral strain E, ν, C, φ, σ R & UCS σ 1 =3 MPa σ 3 =3 MPa u =1 MPa k Kero Flow volume (cc) Time (minutes) k fluid Measure squeeze-out volume σ 1 =22 MPa σ 3 =22 MPa u =10 MPa Flush with kerosene then measure k σ 1 =3 MPa, σ 3 =3 MPa u Up =1 MPa, u Down =0 MPa then set u Up =u Down =1 MPa Bulk volumetric strain (mstr) Effective stress (MPa) Volumetric strain K & σ -porosity Porosity Porosity (%) Pore pressure u Time Bulk modulus (kbar) Effective stress (Bar) Bulk modulus K bulk & α with σ Biot's constant alpha Biot's constant alpha

12 Stress- & strain-path triaxial tests stresses/strains replicate a specific burial or engineering condition q=σ 1 -σ 3 (MPa) Uniaxial compaction (K o triaxial) (i.e. axial compaction with total lateral restraint) p =(σ 1 +2σ 3 )/3 - pore pressure (MPa

13 Conventional K o Test Constant pore pressure σ axial is increased σ lateral adjusted so ε lateral = σ axial (MPa) ε axial (millistrains) σ lateral (MPa) σ axial (MPa)

14 K o triaxial by Depletion/Repressuring Lateral stress (MPa) Pore pressure (MPa) Pressure or stress (MPa) Axial compaction (mstr) Lateral stress Pore pressure Pore pressure (MPa) Time (mins) Compaction (mstr) Elastic compression Onset of pore collapse Inelastic compaction Reservoir stress-path Pore pressure Axial compaction

15 Coupled Tests All these mechanical measurements can be undertake in conjunction with: Ultrasonic measurements Poro-perm (axial and lateral) Electrical properties Rel-perm Cap pressure Temperature Fluid interactions (swelling & core-flood) allowing the investigation of: Coupled properties and behaviour Stress-path and effective stress effects

16 Coupled Ultrasonic/Mechanical (V P, V S1 & V S2, & also transverse transmission) Pulser Typically frequency response 200kHz-1MHz Switch T ducer T ducer Sleeve Triaxial cell Digital oscilloscope Pre- amp PC Printer Switch V P, V S & dynamic elastic moduli at confinement under deviatoric loading, at varying degrees of saturation etc.

17 True Triaxial Testing σ 1 > σ 2 > σ 3 Primarily a research tool Coupled rock mechanical/petrophysical measurement Poro-perm Rel-perm Stress-path etc. Ultrasonic (V P, V S1 & V S2 ) AE Strength Deformation Electrical

18 TWC Studies Overburden Confinement Well pressure Pore fluid Wellbore stability Radial flow Cavity completions Fracture treatments Thermal fractures Perforations Fluid interactions Plasticity corrections

19 Tensile Strength and Fracture Direct tensile test Cylinder rupture test Toughness Brazil indirect tensile test Chevron bend test (also short-rod & notched Brazil tests) Applications : wellbore stability fracture treatment design cuttings reinjection

20 Fracture Properties Stress/pore pressure effects on fracture conductivity Shear and normal stiffnesses Shear strength Residual strength Franklin shear box HPHT triaxial cell with fluidflow along a fracture

21 Differential Strain Analyses (DSA) for in situ stress determination In situ core DSA sample with respect to original core σ A +ve Y Confining pressure σ B Coring Elastic recovery + anelastic deformation +ve X Gauged sample with strain measuring elements at 0 o, 90 o, 45 o & 135 o +ve Z

22 Pressure Crack closure & elastic compression DSA Response Elastic compression Strain Analyses of the crackclosure strains yield estimates of: Principal stress ratios Principal stress orientations Relies on existence of stress-relief microcracks If ASR works, DSA will work Useful adjunct to other techniques Still requires mechanical measurements. Shear-wave splitting, acoustic attenuation etc.

23 Round-Up Experimental measurements are valuable for all studies, i.e., wellbore to far-field, surface to TD Lab measurements provide data that can not otherwise be obtained (i.e., by logs) Coupled rock mechanics/petrophysics studies supplements + complements + enhances SCAL and log derived data Coupled geomechanics and dynamic reservoir simulation means such studies are increasingly needed and are becoming more common