APPLICATIONS ■ Gas-in-place resource evaluation (free

and adsorbed) in cored well ■ Determination of containment

of hydraulic fracture stimulation ■ Maximization of retained-fracture

conductivity ■ Propagation of core-measured field

properties using logs ■ Well-specific completion strategies in

highly variable, heterogeneous systems

BENEFITS ■ Maximized recovery through

● Tailored version of hydraulic fracture design, including proper selection of fracture proppant and fluids

● Assessment of horizontal stress for fracture containment

FEATURES ■ Integration of all data with well logs ■ Capability to resolve

nanodarcy permeabilities ■ Measurements of fluid saturations,

effective porosities, and matrix permeabilities in tight reservoirs

■ Isotherm measurements and canister desorption and adsorption

■ Anisotropic mechanical properties ■ Detailed petrology and X-ray

diffraction (XRD) analysis ■ Fluid sensitivity evaluation ■ Log-based modeling of all above

properties, including petrology

The fully integrated TerraTek TRA* tight rock analysis service characterizes heterogeneity, reservoir quality, and completion quality in unconventional reservoirs.

The workflow includes tasks that are outlined on the right, then detailed on the following pages.

Stress frames are used to conduct tests on cylindrical samples with diameters ranging from 0.5 to 6 in. Tests include uniaxial strain; triaxial, multistage triaxial, and unconfined compressions; and controlled stress path, thick-walled cylinder, and tensile strength. Application of extreme pressure is also available with up to a 60,000-psi confining pressure, a 30,000-psi pore pressure, and a >500,000-psi axial stress (1.5-in diameter sample).

Task 1: Pre-evaluation of log data ■ TerraTek HRA* heterogeneous rock

analysis service ■ Continuous core spectral

gamma ray measurements

Task 2: Petrophysical property identification ■ TerraTek TRA service ■ Total organic content (TOC)

Task 3: Gas-in-place estimation ■ Canister desorption and gas-species analysis ■ Adsorption isotherm analysis

Task 4: Core description and continuous strength profiling

■ Detailed ithologic core description ■ Continuous strength profiling

(scratch testing)

Task 5: Core fracture analysis

Task 6: Petrographic description ■ XRD ■ Thin-section microscopy ■ Scanning electron microscopy of

traditional and ion-milled samples

Task 7: Mechanical property determination ■ Multistress compression test

Task 8: Rock-fluid system service ■ Proppant embedment

and fracture conductivity ■ Permeability reduction ■ Fluid compatibility

Task 9: Analysis and modeling ■ Reservoir quality log ■ Completion quality log ■ Solids production modeling ■ Wellbore stability modeling

TerraTek TRA Tight Rock Analysis Service—Integrated WorkflowFor successful tight gas and oily shale analysis

TerraTek TRA Tight Rock Analysis Service—Integrated WorkflowTask 1: Pre-evaluation of log data for core interval selectionTerraTek HRAservice evaluates the variability of all log measurements as a function of depth, create a pattern of these combined responses, and identify the occurrence of identical data patterns along the length of the well (or the interval of interest using pattern recognition technology). Results from the analysis identify all existing, nonredundant lithologies (rock classes) with distinct material properties along the region of interest.

When performed on nearby offset wells, TerraTek HRA service can be used to identify the distribution of rock units and determine the optimal coring location to represent the most variability. When sidewall cores are taken, log measurements on the same well or from adjacent wells are used to define the depth ranges and minimum sampling requirements to obtain adequate representation. For laboratory sample selection from cores, TerraTek HRA service is used to define the optimal number of samples required for adequate characterization of each rock class (reservoir and seals).

Task 2: Petrophysical property identificationPetrophysical measurements are conducted using TerraTek TRA service to determine reservoir parameters of porosity, permeability, and fluid saturation. These measurements are fundamental for gas-in-place and gas productivity evaluations. TerraTek TRA service uses leading techniques for accurately measuring petrophysical properties of tight gas and oily shales and sandstones. TerraTek TRA service is employed to characterize the gas-filled and effective porosities, the fluid saturations, including mobile hydrocarbons (such as condensates), the as-received matrix permeability to gas, and TOC. Twelve petrophysical parameters are measured and reported.

The combined evaluation of heterogeneous rocks, gas-in-place, geo-chemistry, petrology, and petrophysical results provides a complete definition of the optimal reservoir units in the section logged and defines the adsorbed and interstitial gas in place.

Task 3: Gas-in-place estimationMeaningful estimates of gas-in-place reserves—and subsequent deliverability of this gas—require accurate formation evaluation. The data are obtained by measuring the volume and rate of gas released from recovered core at the wellsite (canister desorption). They are also acquired through laboratory adsorption isotherm and gas-filled porosity and permeability measurements. Lost gas analysis is then conducted to estimate gas in place. Gas analysis and geochemical characterization provide fundamental complementary information for gas composition, organic content, kerogen type, and kerogen maturity. The following tests are specifically recommended:

■ canister desorption analysis ■ desorbed gas analysis ■ elemental logging ■ rock evaluation pyrolysis

Task 4: Core (geological) description and continuous strength profilingDetailed geological description of the slabbed core is essential for defining lithological units and, in combination with petrological analysis, provides the fundamental mineralogical, textural, or diagenetic factors defining the multiple reservoir units and seals. In shales, seemingly subtle variations in mineralogical character, cementation, or deposi-tional environment may result in drastic changes in gas productivity or in mechanical boundaries. These subtle differences may in turn control fracture spacing, distribution, orientation, and conductivity.

The continuous profiling core scratch test system used in TerraTek service provides a continuous unconfined compressive strength (UCS) measurement that is used to map rock heterogeneity and provide a quantitative means of locating intervals requiring further evaluation. The system has become critical in correctly defining facies and hetero-geneities difficult or impossible to observe by geological description or

■ adsorption isotherms ■ vitrinite reflectance ■ TOC ■ free gas porosity.

TerraTek TRA service includes using a pressure-decay permeability measurement system (left) capable of measuring matrix permeability in the nano-Darcy scale in micro-porous materials. Apparent stress-sensitivity measured via pulse-decay on whole plugs (bottom right) due to stress release microfractures (top right). TerraTek TRA service eliminates these artifacts, allowing for more accurate determination of petrophysical properties.

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07 0 1000 2000 3000 4000 5000 6000 7000

Perm

eabi

lity

(nD)

Net Effective Stress (psi)

Pressure Decay Permeability

Pulse Decay Plug Permeability

log characteristics alone. Digital photography allows visual integration of textural heterogeneity and the resulting strength heterogeneity.

TerraTek HRA service and continuous strength measurements along the core are used to determine optimal sample locations for laboratory testing and gain a better understanding of sample-core and core-log scaling relationships.

Task 5: Core fracture analysisCore fracture analysis includes the evaluation of natural and induced fracture systems, fracture orientation, and mineral composition of fracture fill. Typical tabulated data from the fracture analysis include general fracture type, fracture dip orientation, type of mineral fill, type of oil stain, apparent fracture dip, fracture porosity, fracture spacing, and fracture intensity.

Task 6: Petrographic descriptionScanning electron microscope (SEM) analysis, thin-section analysis, and XRD analysis are fundamental to petrological description. Thin-section analyses are used for petrological description of the lithofacies to establish a baseline correlation between the petrophysical and geological and petrological descriptions. They are used as a screen-ing tool for important reservoir parameter investigations such as diagenetic alteration, cementation, and fracture fills.

SEM imaging involves high-magnification imaging of small, representative reservoir samples and is required to identify clay morphology, kerogen location, and pore characterization. Semiquantitative XRD analysis is used to define clay composition and clay expandability and to build log correlations. Certain shale lithofacies can be verified only by XRD analysis. Petrological description from TerraTek TRA service provides textural and mineralogical definitions to the unique log responses identified by the TerraTek HRA service.

Task 7: Mechanical property determinationTo increase recovery of reserves and develop completion strategies, rock mechanics tests are required to assess the variability of elastic properties and in situ stresses. In general, tight gas and oily reservoirs are strongly heterogeneous and anisotropic. As a result, elastic proper-ties are different in the vertical and horizontal directions. This contrast in elastic properties is not directly measured by conventional seismic logs and can have a dominant impact on hydraulic fracturing design and predictions of in situ stress. Therefore, to maximize the effectiveness of completion strategies, the evaluation of anisotropic material properties and the in situ stress throughout the well (e.g., in situ stress variations with respect to reservoir units and seals) are of critical importance.

Fracture toughness and tensile strength are also fundamental parameters required for hydraulic fracturing design and modeling. Measurements of anisotropic material properties, fracture toughness, and tensile strength are conducted on all reservoir units and seals to evaluate the contrast of these properties between lithological units, and thus to evaluate their effect on fracture propagation and containment.

Measurement of the mechanical properties of tight shales requires reservoir effective stress conditions to be applied to the samples. Thus, pore pressure and Biot’s coefficient, which may be much less than unity

in a tight shale, are needed. Biot’s coefficient in tight gas and oily shales, short and long-term creep effects, rock hardness, proppant embedment potential, and proppant fluid sensitivity can all be measured.

Task 8: Rock-fluid system serviceUnderstanding the rock-fluid system (RFS) in unconventional reservoirs is critical for completion optimization. The RFS includes the rock, satu-rating fluids (both hydrocarbons and connate water), soluble minerals, and introduced completion fluids. Fluid flow behavior and rock type are tightly coupled in these reservoirs due to the high surface areas, the very high capillary pressures, and the complexity of both organic and mineral surfaces that exist in these complex pore structures. The RFS package offers multiple services to characterize rock–fluid interactions.

Surface area and pore characterizationGas sorption, mercury-injection capillary pressure (MICP), and cation-exchange capacity (CEC) measurements are used to characterize the surface area and pore structures of the rock.

Native fluids extraction and characterizationServices are offered to extract and characterize the native hydro- carbons in unconventional rocks. Characterization of composition and phase behavior at reservoir temperatures and pressures can be done in conjunction with Schlumberger Reservoir Laboratories.

Connate water characterization scaling potential, salt migration, and mineral dissolution Unconventional reservoirs often contain vast quantities of salt and readily soluble minerals that can dissolve and diffuse into the introduced fracturing fluids. Their variety of salts and minerals can be fully character-ized, and full flowback fluid analysis is available from the Schlumberger water analysis laboratory.

19,000

15,200

11,400

Axial stress difference, ×1E4 psi

7,600

Ultra

soni

c ve

loci

ty, f

t/s

3,800

0 0

1.2

2.4

3.6

4.8

Axia

l stra

in, %

6

–0.5 0 0.5 1 1.5 2 2.5 3 3.5

P waveS waveAxial strain

Triaxial compression depth: 7,453.05 ft

Axial stress is measured versus strain during triaxial compression testing. Ultrasonic velocity measurements of P and S waves are captured during loading up to the point of failure. Measurements allow the characterization of static and dynamic elastic properties and the stress dependence of dynamic properties.

TerraTek TRA Tight Rock Analysis Service—Integrated Workflow

TerraTek TRA Tight Rock Analysis Service—Integrated Workflow

Chemomechanical effectsNumerous chemomechanical tests are routinely performed to measure

■ loss of fracture conductivity due to proppant embedment ■ loss of unconfined compressive strength (UCS) due to fluids exposure ■ sensitivity of swelling clays ■ rock failure and fines generation.

Testing can be performed with a variety of completion fluids and at reservoir temperatures and pressures. Flowback pressure management testing and modeling services are also provided.

Fluids retention and migrationThe vast majority of the fluid pumped during fracturing treatments stays in the formation. Where does it go? What does it do? Services include

■ quantitative imbibition ■ quantitative fluid leakoff ■ advancing and receding contact angles used to predict fluid imbibition

into the rock and the retention of fluid in the fracture.

The RFS service package focuses on quantitative testing methods to generate data that can be incorporated in reservoir simulation models. All tests can be integrated into comprehensive custom service packages along with geomechanical, petrological, core analysis, and HRA services to provide a complete and detailed understanding of the rock-fluid system in these reservoirs.

Task 9: Analysis, modeling, and predictions of properties with logsPre-evaluation of log data using TerraTek HRA service provides a math-ematically precise, subjectivity-free, and robust methodology. The process defines rock classes with distinct material properties and propagates in situ stress profiles for fracture design. Tight rock and core analysis and laboratory testing provide the fundamental petrologic, physical, and mechanical properties for a comprehensive characteriza-tion of each of these rock units. Well-defined rock units and their well- characterized properties and log responses are used to develop a model that predicts log responses alone. A direct result of this analysis is the ability to predict all measured laboratory properties in subsequent wells based on log measurements alone. Additional analysis of selected properties provides continuous estimates of reservoir quality (e.g., gas productivity potential) and hydraulic fracture containment across the field.

Texture, distribution, and condition of organic components is revealed by SEM imaging.

Thin-section analysis, in combination with XRD data, is used to obtain matrix composition (<4 microns), microtexture, amounts and compositions of silt and microfossils, and diagenetic textures. Here, fossil composition is determined by colored stains.

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