For developers of US shale plays, economic viability depends on technologies and methods that can not only enhance understanding of complex, self-sourcing reservoirs, but also maximize operational efficiencies at each stage of development. Accurate, real-time data from logging while drilling (LWD) can play an important role in identifying stratigraphic sweet spots, placing the well within the target interval and positioning frac stages to maximize production rates and recovery.

Unique attributes drive unique approach

Shale plays are unique in that they serve as both the source rock and reservoir rock for hydrocarbons. They tend to cover large land areas and lack the gas- and oil-water contacts that characterize conventional reservoirs. They exhibit dramatically lower permeability and a larger content of organic matter. Unlike conventional reservoirs, where matrix porosity is the primary mechanism for gas storage, shale reservoirs also contain significant amounts of gas adsorbed onto the organic matter (kerogen) and mineral surfaces.

While the presence of hydrocarbons in organic shales often is pervasive, the challenge is to identify the so-called sweet spots, where favorable parameters such as thickness, porosity, permeability, organic content, mineralogy, brittleness, natural fracturing, thermal maturity, and gas content come together, and then match those parameters to the right drilling, completion, and stimulation methods and technologies to optimize economic production.

The depositional environment of most organic shales results in considerable textural, compositional, and geo-mechanical heterogeneity. The thinly-bedded nature of the deepwater deposits and the tectonic forces applied during the subsequent burial often result in significant petrophysical and geomechanical anisotropy. Abrupt changes can occur over short vertical and horizontal distances, resulting in localized changes in permeability and brittleness that can help explain significant differences in production rates between adjacent wells.

Real-time azimuthal gamma ray LWD logs and borehole images help determine dip angle and detect bed boundaries that enable geosteering engineers to keep the well in the sweet spot throughout long lateral sections. (Images courtesy of Weatherford International)

For these reasons, defining and understanding the differences between (and even within) shales is more important to successful development than identifying their similarities. This understanding requires that conventional formation evaluation technologies and methods be enhanced with specialized systems and services. Whereas geology, petrophysics, and geophysics comprise typical formation evaluation methodologies for conventional reservoirs, analyses of mineralogy, geomechanical, and geochemical properties must be added to the mix for shale plays to increase understanding of critical properties such as rock strength, stresses, total organic content (TOC), and thermal maturity. Many geoscientists concur that, in shale plays, geometry is as much an issue as gas content, natural storage capacity, and the rock’s deliverability.

Learning from LWD

Economically viable shale development requires a delicate balance between controlling costs and optimizing production over the life of the reservoir. While the primary focus of wellbore construction operations is on wellbore placement and optimizing the completion, success depends on understanding the geological, geomechanical, and petrophysical properties of the reservoir and well and how they impact drilling, completion, and stimulation operations.

LWD is used to help characterize potential reservoir targets and to provide critical information for optimizing well placement and evaluating rock properties for fracturing. The real-time nature of LWD measurements means critical information can be provided to well teams as the well is being drilled, thus making it crucial for geosteering the well to the sweet spot and keeping it there, while reducing non-productive time (NPT).

A conventional triple-combo LWD suite consists of gamma ray, resistivity, azimuthal density, and neutron porosity sensors. The data from these sensors are used to determine petrophysical properties such as lithology and porosity, while real-time borehole images provide formation dip information and facilitate proactive geosteering to place the wellbore in the optimum stratigraphic position. LWD is integrated into the bottomhole assembly with a rotary-steerable system (RSS) for geosteering.

Some shale reservoirs, such as the Barnett in north-central Texas, are relatively thick and unfaulted. For these reservoirs, basic triple-combo LWD can provide the data needed to characterize the formation and design completion programs.

The Haynesville shale, which spans approximately 9,000 sq miles (23,309 sq km) across northern Louisiana and East Texas, exhibits significantly higher reservoir pressures than other shale plays and downhole temperatures that can reach 300°F to 370°F (149°C to 188°C). To address these conditions, specially engineered RSS and LWD systems, such as Weatherford’s HEL hostile-environment logging systems, are required.

In a number of shale plays, such as the abundant Marcellus and Eagle Ford, challenges exist that cannot be properly addressed with basic sensors. The Marcellus, for example, is approximately one-half the thickness of the Barnett, with lower gas content. It is highly folded and faulted, and poses additional challenges by its proximity to large population centers in the northeast US. Local variations in mineralogy, TOC, clay type, and thermal maturity also impact drilling performance, well placement, and fracture design. The Eagle Ford in south Texas is unique in that it produces natural gas, oil, and condensate. Its geology and geochemistry are highly variable, particularly with regard to pore pressure, degrees of natural fracturing, and brittleness, which increases significantly across the play in relationship to local increases and decreases in clay and calcite content.

Enhancing understanding

Achieving the degree of geological and petrophysical understanding necessary to sustainably develop prolific but highly complex plays like the Marcellus and the Eagle Ford requires additional information with advanced LWD sensors, such as spectral azimuthal gamma ray and sonic.

The spectral azimuthal gamma ray (SAGR) LWD sensor helps determine dip angle and detect bed boundaries that enable geosteering engineers to keep the well in the sweet spot throughout long lateral sections. It also provides potassium, uranium, and thorium measurements for real-time TOC evaluation, as well as clay content and typing.

Real-time azimuthal gamma ray LWD logs and borehole images help determine dip angle and detect bed boundaries that enable geosteering engineers to keep the well in the sweet spot throughout long lateral sections. As the industry’s only spectral azimuthal gamma ray LWD sensor, Weatherford’s SAGR tool also provides potassium, uranium, and thorium measurements for real-time TOC evaluation, as well as clay content and typing. The common correlation of LWD-measured uranium content with kerogen volume, TOC, and gas-in-place provides a new real-time dimension for steering horizontal wells into the most productive part of the formation and evaluating the production potential of lateral shale wells. With this advanced petrophysical information, well trajectories can be pinpointed to enable successful selective zonal fracturing that can reduce stages and costs.

Sonic LWD sensors such as the ShockWave sonic tool provide real-time seismic time-depth correlation and provide rock mechanical properties, including Poisson’s ratio and Young’s modulus, which are key to positioning frac stages for optimum results, planning the hydraulic fracture operation, and modeling production behavior, even in acoustically and mechanically anisotropic formations.

Balancing cost and technology

As with other oil and gas play types, economic viability and sustainability of shale plays is a balancing act. The difference is shales have lower margins than other play types. Shale play economics also are impacted by the fact that development programs must not only enable prolific initial production rates, but also must foster transition to maximum EUR from low-decline producing wells with typical life expectancies that can exceed 30 years. Sustainability requires continuous efforts to improve efficiencies in drilling, completion, and fracturing operations while reducing NPT. LWD is proving to be a key enabling technology for creating and maintaining the shale cost-technology balance.