Our intern needed an education on what we do here at Bluware, so we asked him to find a technical article on oil and gas exploration. He found this post, and now thinks he’s an expert on GGI. Show him how much he has to learn in the comments.
Providing an adjunct to seismic, gravity gradiometry images structural complexity at a fraction of the cost of a 3-D survey.
With shale gas having the potential to change the demand/supply energy outlook in North America, today’s operators are looking for ways to exploit the resource as efficiently as possible.
Identifying, mapping, and staying within sweet spots determining well locations and spacing so drilling can be optimized and designing the most effective production strategy, while taking heed of environmental sensitivities, are all key drivers for shale gas operators today.
Rising to the challenge
There is no doubt that today’s 3-D seismic technologies are capable of developing highly accurate 3-D structural models of shale gas fields, helping to optimize drilling and completions.
Advanced seismic processing and high-resolution wide-azimuth 3-D surveys are generating detailed information on key reservoir properties such as pore pressure and local stresses and are providing important input for fracture stimulation programs.
While information generated from seismic is crucial in guiding drilling and completion programs, gravity gradiometry imaging (GGI) also is playing an important role alongside 2-D seismic in optimizing exploration strategy in shale gas plays.
These benefits are principally seen in two crucial areas – firstly, in identifying zones where there is a high probability of structural complexity, which can subsequently have a negative effect on production and fracing programs and secondly, in reducing cost and risks surrounding shale gas exploration.
Airborne GGI surveys are able to explore vast regions quickly and efficiently with minimal environmental impact and with significant implications for reducing risk and cost (typical multi-client surveys in North America cost in the region of US $2 an acre, whereas 3-D seismic can cost as much as $150 an acre).
Modeling complex geologies
GGI is a fast growing technique that maps small density variations in underlying rocks by measuring the gradient of the earth’s gravity field.
The technology benefits of GGI in traditional oil and gas exploration have significant applications in shale gas fields today. For example, the high-resolution data and bandwidth generated, coupled with the strong signal-to-noise ratio, make GGI a highly effective technology in modeling complex geologies and generating accurate velocity/density relationships.
Through precise positioning of subsurface density contrasts, GGI can accurately target geological settings, derive basement structure maps, and provide input to the structural definition of overlying sedimentary sections and horizons.
The ability for GGI to include 3-D geology in its datasets allows the seismic interpreter to develop structural models outside the plane of 2-D seismic acquisition and provides an increased spatial awareness of the target area.
GGI can provide valuable data to interpolate between two seismic lines – often several miles apart – providing confirmation of whether two independently interpreted faults on two seismic sections are connected. This is a significant benefit in the early stages of shale gas exploration where 3-D seismic access is restricted. Here, GGI’s ability to look at the geology “sideways” when developing its structural models is of high value to shale gas operators.
The most significant application GGI has for shale gas exploration is through a new solution called ShaleQube. This exploration workflow combines the structural modeling information generated by GGI with the finite element method, which is used to calculate zones where there is a higher or lesser probability of structural complexity within a target sequence.
With such structural complexities increasing the likelihood of the operator losing contact with the shale or coming across faults that have the potential to divert hydraulic fracturing or reduce pressures (ultimately resulting in abandoned wells), the information generated from GGI and the resultant finite element analysis (FEA) can be crucial in determining exploration strategies.
It is more important than ever for operators to understand the structural complexity of shale gas fields, the presence of faults, and which areas of the field are more or less structurally complex than others ahead of planning 3-D seismic acquisition.
GGI on the Montney
The Montney formation is in the Western Canada Sedimentary Basin in British Columbia and Alberta. Here, shale gas extraction began in the late 2000s.
ARKeX carried out an initial GGI survey in partnership with JEBCO Seismic LLP in 2005 and 2006 to identify a Mississippian gas play. At the time of acquisition, the Montney shale gas play was not a prime target, so the survey design was not optimized for this play type. However, the data did cover part of the Montney shale and subsequently were used to help develop the ShaleQube workflow.
Using the GGI interpretation, an isopach map was generated that clearly showed the sedimentary structure within the Montney rifted section and the faults believed to control fluid distribution. The structural data were then input into FEA, where the combination of high-resolution GGI data plus a 2-D seismic-guided 3-D earth model generated a 3-D volume of the structural complexity of the shale gas interval.
In this case, zones were highlighted where there was an increased probability of faulting and where the operator might want to acquire further 3-D seismic data. The FEA-derived model illustrated more structurally complex areas where there was a high probability of encountering faults and less complex areas where there was a lower probability of encountering faults.
The model provided crucial information for operators making exploration decisions on shale gas plays and the targeting of future 3-D seismic surveys.
GGI on the Marcellus
ARKeX also has been working in partnership with Global Geophysical Services to improve understanding of the Marcellus shale&rsquos structural complexity on a regional scale. The Marcellus shale is one of the largest sources of domestic natural gas to be discovered to date in the US.
The airborne survey of 9,065 sq km (3,500 sq miles) was completed in just 100 days, and the preliminary GGI data are providing valuable information about how the structures and faulting, seen in 2-D seismic data, can be linked to help provide an overall understanding of the structural framework in the area.
The data show a strong correlation to the fold axes interpreted in the region where low-density regions exist along the anticlines and high-density regions where synclines are present. While the GGI data do not image the Marcellus shale itself, they are able to identify the density interfaces that influence the Marcellus, particularly the salt the shale is sitting on top of and the Tully and Onondaga limestone layers that bound the Marcellus interval.
Although the survey results need to be analyzed in more detail, an improved understanding of the Salina salt and associated faulting will provide valuable information about the structural complexity within the Marcellus.
Getting in on the ground floor
Although 3-D seismic is still playing the lead role in mapping out shale gas reservoirs today, if used at an early stage in the exploration lifecycle, ShaleQube can play an invaluable role in identifying key structural elements such as faults, identifying more structurally complex zones, providing input into the design and targeting of future 3-D seismic surveys, and optimizing drilling program and production strategies.