A better definition

Aug. 3, 2015
The growth of unconventional resources in the last 20 years is largely attributed to advanced tools and drilling techniques.

Tayvis Dunnahoe
Exploration Editor

The growth of unconventional resources in the last 20 years is largely attributed to advanced tools and drilling techniques. Another important factor that has increased the amount of recoverable reserves in most unconventional reservoirs is a better understanding of the subsurface in many of these plays.

As a life cycle, seismic, exploration drilling, and development share a symbiotic relationship. The value of each facet of this process varies with which perspective is driving the conversation. Business decisions are made based on what an operator may know about a particular drilling target. Considerations are given to not only the geologic understanding, but also on the availability of equipment, tools, experienced manpower, and access to infrastructure.

The question remains, "Which of these elements take priority in the decision to drill?"

According to a recent survey conducted jointly by Oil & Gas Journal and PennEnergy, operators selected geologic data as the most important factor in planning a well.

The survey, Unconventional Well Business Processes: Operator & Non-Operator Perspectives, polled both operators and non-operators to define primary business considerations in developing unconventional resources.

With the primary objective to increase ultimate recovery, participants were asked to rank the various processes involved in developing a well. The highest ranked factor among all respondents was data. Operators, which accounted for 39.6% of respondents, ranked geologic data higher than data on chemicals and proppants. The consensus was somewhat polarized with -0.39 index among operators and non-operators.

While subsurface tools were ranked as the second-highest priority, the survey revealed that better understanding of the subsurface would lend more credence to investing in a well.

Gaining a better understanding of geologic, petrophysical, and fracture propagation data also ranked high as one of the biggest challenges faced in acquiring the skills to design a well. Subsurface visualization technology was selected as the area where more technological enhancements were needed. In an open-ended question inviting respondents to offer ideas that would improve well design, 31% said data analysis should become more advanced. Reservoir visualization and modeling came in behind this with 16% of respondents providing ideas.

New ideas, not new rocks

Unconventional development, namely horizontal drilling, has come full circle in the US in nearly 30 years. In 1990, OGJ described drilling activity in Pearsall (Austin chalk) field in South Texas as "hysterical" after the area saw 40 rigs operating from almost no rigs the previous year (OGJ, Feb. 26, 1990, p. 53).

Pearsall field was considered "a good case study" for horizontal drilling in historic fields. The area had seen almost 2,000 wells since 1936. More than half of the new drilling in 1990 spawned from advances in hydraulic fracturing in the mid-1970s.

In 1995, a different survey cited the pace of horizontal drilling in the US at about 1,000 well/year. Of these, 79% were drilled in the Austin chalk (OGJ, June 19, 1995, p. 39). At that time, about 90% of all US horizontals were drilled in carbonate formations. The most common application for horizontal technology in the US was intersecting vertical fractures (53% of all formations).

Horizontal drilling was not a new process, but the Austin chalk provided a proving ground that serves as the basis for today's shale development.

Defining the subsurface

Unlike most carbonate formations, the later shale boom would often require operators to manufacture porosity in low-permeability environments using advanced fracing techniques. In its earliest phase, shale development was primarily a trial-and-error process though with errors coming at great cost.

Improvements in data acquisition, microseismic, and 3D visualization have paved the way for tailor-made drilling and completion applications in many shale plays within the US as well as improved reservoir modeling and sweet spot identification. With ultimate recovery as a goal, it stands to reason that geologic data remain an important factor in deciding to invest in an exploration well. While the subsurface is static, the industry's interpretation of unconventional reservoirs provides better definition.