The discovery of commercial oil and gas production from shale, or mudstone, reservoirs has dramatically changed how we explore for and
develop oil and gas accumulations. In conventional exploration, appraisal and development there is a fairly standard and accepted application
of processes and technologies. However, the processes and technologies that are employed in the exploration, appraisal and development of
mudstone reservoirs are significantly different, and they are often employed for different reasons and at different stages of the cycle.
Prospect identification is always the initial phase of any exploration project. In most cases in the conventional world this is a result of the
interpretation of seismic data, either 2D and/or 3D, in order to identify
the areal extent of the prospect, which would typically be on the order of
a few hundred acres or, in some instances, a few thousand
acres. However, in the unconventional world the identification is done at a basin
level and is typically not supported initially by seismic,
but rather by detailed analysis of a few key wells and their associated
petrophysical
attributes. Once those attributes are deemed to
have the potential of supporting a commercially productive mudstone reservoir, then the
utilization of seismic is employed to help define the boundaries of the reservoir, understand the structural components of the basin and, in many
instances, preliminarily map the
thickness of the reservoir. The third preliminary source of data that would identify the quality of the prospect
would be a geochemical analysis of the reservoir. While this can often be difficult to obtain
due to the lack of core or drill cuttings to obtain the
analysis, it can
be a very critical component of the identification process.
Once the prospect has been identified, the evaluation processes during the exploratory drilling phase are dramatically different. During
conventional exploration the determination as to whether hydrocarbons
are present is largely done by the acquisition and interpretation
of data
from openhole wire line logs. While cores, either whole or sidewall, will often be taken, they are typically not acquired to validate the
productivity of the reservoir, but rather to supplement the openhole log data. In unconventional exploration, the opposite is the case. While the
openhole logs are extremely important once the discovery is made to calibrate the reservoir, the most critical data
around the validation of the
quality of the reservoir is the detailed analysis of the rock acquired fro
m whole core. While some of the attributes that are measured from the
mudstone core are common to conventional exploration, there are many more measurements that are taken on mudstones that are totally unique
to this type of
reservoir.
As the prospect moves into appraisal and development mode, there
are also unique processes and technologies in the unconventional world that
are used to more fully understand the reservoir. The most
important of those is the calibration, through the use of specific algorithms, of the
data acquired from the whole core data to
the openhole data that is being acquired from the appraisal and development drilling. Because the
cost and time necessary to acquire an extensive collection of whole core data can be prohibitive, there will be a limited number of wells from
which whole core is taken in any given field. Therefore, it is critical to be able to calibrate the various measurements from the whole core to the openhole log data that will be available on many more wells. This is
also the point during which 3D seismic would be acquired, as opposed to
the acquisition of that type of data during the identification process in conventional exploration. In unconventional development, the primary
benefit of the 3D seismic data is not to identify where you want to drill, but rather where you don’t want to drill. Specifically, the horizontal
lateral is placed to minimize the effect of faulting on the lateral.
Throughout the entire period of field appraisal and development, the practice of geosteering is critical to the economic success of the field.
Since virtually all of the unconventional development is done with the application of horizontal drilling, it is critically important that the drill
bit maintains its position within the
identified target window while the lateral is being drilled. This target window can be within the section
where the highest quality reservoir h
as been identified, within the optimum stratigraphic position of the
reservoir in order to optimize the
completion, or, in most cases, a combination of both. Since the
drilling operations are performed around the clock, and unexpected changes in
dip or the presence of faults can cause the bit to rapidly change its relative stratigraphic position, a
Gamma Ray tool is incorporated into the
bottom hole drilling assembly in order to provide continuous measured depth Gamma Ray log data, which is then converted to a true vertical
depth (TVD) log using software designed specifically for this process. This TVD log data is subsequently correlated with nearby well control
to determine where the lateral is positioned stratigraphically at all times during the drilling operation. When the bit has been interpreted to be
out of t
he desired target window, it is the responsibility of the geosteerer to collaborate with the drilling organization to make the necessary
changes to get the bit back into the target window.
While the previous example of the role of the development geologist is focused on the day to day operations of the field, one of the most
critical inputs that they have is in the long range development of the field. Due to the extremely large area that these fields typically encompass,
the early establishment of unit spacing and unit configuration is critical to minimize the amount of spoiled acreage that will occur as a result of
faulting, surface limitations and the complexity of integrating land not under your control. This is a multi-disciplined effort between geology,
land, and engineering, but the lead must be taken by the development geologist in order to maximize the value of the asset.
All of these processes and technologies have truly transformed how
geoscience is applied to the exploration and development of mudstone
reservoirs. Additionally, considering the fact that the exploration and development of mudstones is s
till relatively immature, it is likely that
many more changes will be developed as the science matures.