NXT Energy Solutions Inc. (NSFDF)

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Insight: Value of Stress Field Detection technology in early-stage exploration

Friday, September 06, 2013
Findingpetroleum.com

Overview of Stress Field Detection (SFD®)
The SFD® system utilizes quantum-scale sensors to detect gravity field perturbations which are induced by terrestrial stress energy variations, primarily in the horizontal plane. Significant subsurface discontinuities or anomalies are inherently associated with, and dependent on, subsurface principal stresses. As a consequence, the discontinuities will distort stress fields, resulting in a unique in-situ stress pattern.

SFD® has been used by oil and gas companies to identify subsurface stress-field anomalies related to trapped fluids. This technology offers a leap in cost-effective rapid reconnaissance for early-stage exploration from an airborne platform.

Heuristic Explanation
The subsurface geological condition required for SFD® to detect gravity field perturbations which arise due to stress variations in the horizontal direction is the occurrence of a structural and/or stratigraphic change discontinuity with sufficient difference in elastic properties. An important source of elastic variations is the presence of trapped fluids (oil, gas, or water). Other sources include faulting / fracturing, over-pressure, major lithological changes and basin boundaries. In general, all major discontinuities will evoke a distinct SFD® response. The variation of the shear component around and in reservoirs will result in the redistribution and orientation change of the stress fields (Figure 1).

Figure 1. Plan view of principal horizontal stress components (SHmax and SHmin) illustrating the distortion of stress fields associated with fluids trapped in a structural thrust-fault scenario. Stresses are redistributed due to a variety of sources including porosity, trapped fluids, fracturing, faulting and reservoir pressure.

Differences with Conventional Gravity and FTG

Conventional gravity surveys measure the vertical component of the gravitational field. Full tensor gravity (FTG) gradiometry measures the rate of change of all components of the gravity tensor. Two major advantages of FTG over conventional gravity are the increased resolution and the reduced noise contamination of the signal. Both methods rely on a large test mass to increase the sensitivity in measuring subsurface density variations.

Both conventional gravity and FTG provide a static, point to point measurement. However, SFD® differs from both in that it employs particle scale sensor elements for dynamic detection, which enables selective sensitivity to directional changes in subsurface stress induced by poro-elastic anomalies.

Case Studies

Gulf of Mexico: Integration of Regional Geophysical Data with SFD®
The Gulf of Mexico has been an area of evolving exploration: from the continental shelf to salt-controlled basins to sub-salt and ultra-deep water plays. Covering vast areas in a cost-effective manner to identify prospective acreage is an ongoing challenge. Conventional wisdom has been to conduct regional 2D seismic surveys followed-up with 3D seismic, both sparse and detailed. Although seismic data provide high-resolution subsurface images the cost is significant. Airborne SFD® can provide a cost-effective rapid reconnaissance solution that identifies prospective areas for more focused follow-up seismic efforts.

In the Fall of 2012, PEMEX conducted an initial SFD® survey in onshore and offshore areas of the Gulf of Mexico region. Figure 2a shows a 2D seismic section from the Gulf of Mexico. The sub-salt structure appears to be encased in a sealing salt (purple) but there may be a breaching potential due to the marked faults.

The anomaly identified on seismic is verified by the SFD® anomaly (Figure 2b). This may be indicative of the presence of both trapped fluids and an effective sealing mechanism.The correlation of SFD® with seismic poses that in areas without seismic coverage, an SFD® survey could help propose where to put in a capital-intensive high-resolution seismic program.

Figure 2. Correlation of regional 2D seismic data with SFD®: a) Seismic anomaly PMX-2.16 and b) corresponding SFD® signal.

Furthermore SFD® is effective irrespective of the presence of salt or bathymetry. As such, SFD® can serve as an important complementary method to be used in conjunction with other exploration technologies.

While seismic surveys offer excellent resolution and structural definition, they do not always easily lend themselves to identifying fluid properties in the absence of additional information. New methods, such as controlled-source electromagnetics (CSEM), are becoming more accepted tools to investigate fluid indicators on qualified prospects. Figure 3 illustrates another Gulf of Mexico example correlating SFD® with CSEM high-resistivity anomalies associated with hydrocarbons. Figure 3a is a map of CSEM resistivity overlain by an SFD® survey line. There are two high-resistivity anomalies (dashed red ovals) identified as PMX-2.44 and PMX-2.15 which have corresponding SFD anomalies (solid red bars). The SFD® profile for PMX-2.44 is shown in Figure 3b. In this example, both CSEM and SFD® have identified potential fluid anomalies.


Figure 3. Correlation of regional CSEM data with SFD®: a) CSEM resistivity anomaly map with dashed red ovals Indicating high-resistivity anomalies at PMX-2.44 and PMX-2.15 and b) corresponding SFD® signal over PMX-2.44.

Colombia: Optimizing Lease Relinquishments with SFD

A common exploration challenge is to determine the best acreage to retain within a large lease. In 2010 NXT Energy Solutions conducted an airborne SFD® survey covering 7,500km2over the Tacacho and Terecay Blocks in Colombia for Pacific Rubiales.Figure 4a shows the approximately 7,000 line km of SFD®data (red lines) which were acquired, processed and interpreted to provide an evaluation in less than two months time. As a result of this rapid turnaround, the leaseholder was able to identify the areas to be relinquished. Figure 4b shows the retained areas in red outline. The SFD® anomalies are indicated in the blue boxes and the green outlines represent the client’s assessment of anomaly areas using other technical data acquired subsequent to the SFD® survey.


Figure 4. High-grading blocks for exploration: a) Blocks with flight lines (red) and existing seismic lines (black) and b) blocks containing the best SFD® anomalies and retained acreage outlined (red boxes). SFD® anomalies are identified with blue boxes and the clients assessment of anomalies using numerous G&G methods are indicated by green outlines.

Benefits for Early Stage Exploration

Rapid and efficient exploration reconnaissance over large areas of virgin acreage in an environmentally sensitive manner
Identification of prospect areas with potential trap, reservoir quality and seal integrity
Effective in both onshore and offshore environments
Insensitive to both water depth and the presence of salt
Prioritize exploration activity (e.g., seismic programs) on identified leads

Author: George Liszicasz, President & CEO
Company: NXT Energy Solutions Inc.

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