NATURAL GAS:
Part 6: Anomalously pressured zones

Anomalously pressured gas accumulations in Rocky Mountain Laramide basins

Case history describes application of new exploration / exploitation approach in area with non-conventional geology

Ronald C. Surdam, President, Innovative Discovery Technologies, LLC, Laramie, Wyoming

he following discussion provides a definition and background of anomalously pressured (AP) reservoirs and the vast potential of this underdeveloped resource. Challenges facing engineers in drilling and completing wells safely and efficiently are outlined. And a new, innovative exploration / exploitation approach developed by Innovative Discovery Technologies, LLC (IDT), of Laramie, Wyoming – a new Gas Technology Institute subsidiary, created in partnership with Surdam Consulting, Inc. – is introduced and illustrated with a case history application. The application is an example of the type of detailed information required to "intelligently" develop AP gas assets.

Introduction

Anomalously pressured gas accumulations in Rocky Mountain Laramide basins represent a huge, yet underdeveloped energy resource. For example, in the Lower-48 states in recent years, only Wyoming has maintained a positive slope on its gas production curves, i.e., new gas discoveries are outstripping reserve depletion. This positive slope in Wyoming has largely resulted from discoveries / developments at Jonah and Cave Gulch fields, both giant, AP gas accumulations.

Of course, future development of coal-bed gas will also enhance the positive slope on the state’s gas production curve. Thus, in Wyoming, it has been development of "unconventional" gas resources that has reversed the highly undesirable, but all too real, energy depletion curves plaguing domestic U.S. production.

So-called unconventional, basin-center, or deep-basin gas accumulations all have one characteristic in common – anomalous pressure. Basins characterized by relatively continuous burial tend to be predominantly overpressured, whereas basins characterized by more complex burial histories that include episodes of uplift may be under- or overpressured; or they may contain rock-fluid systems both under- and overpressured. The subject of this article is anomalously pressured gas accumulations, i.e., rock-fluid systems in which the pressure plots off the hydrostatic gradient, regardless of what pressure nomenclature or basin one uses.

The working hypothesis presented is as follows: the quickest way to reverse the negative slope of domestic gas production curves is to more effectively and efficiently exploit AP gas accumulations. The magnitude of the energy resource is sufficient. In the Wind River basin of Wyoming alone ( 8,000 mi²), the USGS estimates that the in-place AP gas resource is about 900 Tcf; yet cumulative production from the stratigraphic units containing this resource, to date, has been less than 1 Tcf. Thus, the energy resource is in place, but in most cases, lack of essential new exploration / exploitation technologies and techniques has constrained development.

AP gas accumulations offer exploration scientists / engineers a special set of challenges. If these challenges are met and overcome, essential new technologies will be created and evolve, resulting in ever-increasing conversion of these resources to energy reserves. Some, but not all of these challenges are discussed below.

Converting AP gas accumulations to viable energy reserves has been a particularly expensive task for explorationists in general. Initial efforts to exploit these resources have been disappointing and very costly, largely because traditional exploration and completion techniques have proven to be relatively ineffectual. Conventional exploration techniques fail because AP gas accumulations are not necessarily confined to structural or stratigraphic traps.

Jonah field provides an excellent illustration of this point. Isopach and structural contour maps of the field¹ clearly show that neither structural closure nor stratigraphic pinch out characterizes Jonah, although faults form a "structural trap" as lateral seals to the field. Not only is structural closure absent, there is no apparent indicator of the field on 2-D seismic amplitude stack profiles across the field.¹ Both the west- and south-bounding faults (two of the field boundaries) are extremely difficult to discern on the seismic stack. Thus, Jonah presents and illustrates the problems facing explorationists when searching for AP gas assets.

Another problem that is particularly vexing to explorationists is that many anomalously pressured gas accumulations occur structurally down dip from water. In other words, the AP accumulations occur below the conventional water leg in many basins and may be part of a gas-charged column many thousands of feet high. In these cases, the rock-fluid systems are compartmentalized, with the upper, conventional fluid-flow system separated from the underlying, AP fluid-flow system by a regional pressure surface boundary.²

The most important task in resolving these problems is to reduce uncertainty about the distribution of AP gas within the basin. Once gas distribution is determined, the next significant step is to detect and delineate porosity / permeability sweet spots (areas of enhanced storage capacity and deliverability) below the regional pressure surface boundary.²

Drilling

The challenges facing engineers designing drilling strategies for anomalously pressured gas accumulations are equally daunting. Not only are there obvious safety concerns when drilling across the regional pressure surface boundary into an AP fluid system, but the characteristics of such a rock-fluid system can change abruptly and dramatically. The most significant change occurs from a single-phase fluid flow system, e.g., dominantly water, typically under strong water drive, to a multiphase fluid-flow system, dominated by capillarity.

This transition is accompanied by a pressure change from normal to abnormal pressure, which is often abrupt. Moreover, the change can be from normal, i.e., hydrostatic, to significant overpressure or underpressure. The driller must design a strategy that takes safety issues into account when moving from normal to anomalous pressure regimes – but that still minimizes formation damage. Whether one drills into under- or overpressured, capillarity-dominated fluid systems, the rock-fluid system is extremely sensitive to introduced fluids and, hence, mud weights. Ideally, the drilling design should include an attempt to safely drill with an undercompensated mud program, and with fluids compatible with the penetrated capillarity system – a monumental problem.

Completion

Completing wells in anomalously pressured terrains presents equally perplexing problems. In many cases, the first step in completion is to mediate formation damage acquired during drilling. Experience has demonstrated, particularly with respect to "tight gas sands," that introduced water must be avoided during completion techniques. Also, remnant water in the reservoir sequence must be avoided during fracture completions.

Excessive water production is a sure way to kill an AP gas well. Extreme care must be taken to avoid fracing into AP domains characterized by significant volumes of remnant connate water. Because AP gas accumulations are highly compartmentalized, limited-entry fracs and water avoidance are highly desirable.

Innovative Exploration and Exploitation Approach

A new exploration / exploitation technology has been developed that overcomes many challenges in searching for AP gas assets. The IDT technology, developed by Surdam and Associates, significantly and quantifiably reduces the risk of exploiting gas assets in low-permeability formations. Application of the IDT technology provides the operator, prior to drilling, with the following essential information: 1) spatial distribution of gas-charged rock / fluid systems, including pressure compartment boundaries; 2) location and nature of the regional pressure surface boundary, or boundary between normally and anomalously pressured fluid systems (either under- or overpressured); 3) determination of gas and water content in the fluid; 4) identification / spatial distribution of microfracture swarms; and 5) orientation and timing of faults.

Only with such specialized information will optimum drilling and completion practices be designed / employed that enable operators to avoid a multitude of disastrous pitfalls currently associated with exploration in AP, low-permeability formations, such as excessive water production, formation damage, and a wide range of problems associated with capillarity and "incompatible" fluids. These problems, heretofore, have inhibited corporate enthusiasm by limiting profitability.

The new technology has been rigorously tested and validated with both public data (Riverton Dome DOE Demonstration) and proprietary data (Jonah field), and in 28 other basins around the world. Armed with the technology, explorationists and engineers are able to more effectively and efficiently detect / delineate commercial production sweet spots in low-permeability formations, and avoid excessive water production by designing optimum drilling / completion programs, thereby greatly increasing gas production from low-permeability formations.

Case History

To illustrate in more detail how the IDT technology operates, consider the following case history for an AP gas accumulation. This case history is from an area where there is a total lack of structural closure and no stratigraphic pinch outs with respect to the targeted reservoir interval. Several nearby sonic logs from unsuccessful wells indicate that the targeted reservoir interval in the area of interest is overpressured and relatively tight. The anomalous velocity volume shows that a significant velocity anomaly, i.e., velocity slowdown, occurs at the level of the potential reservoir interval, Fig. 1.

Fig. 1. Gas distribution. Anomalous velocity map sliced at depth of reservoir interval of interest. Red area is characterized by anomalous velocity values falling 1,600 to 2,000 meter/sec below (or slower) than would be predicted by typical regional velocity / depth gradient at this depth. Intensely slow domains are the result of presence of a significant ubiquitous free gas phase in the fluid.

In addition, it is apparent from an anomalous velocity cross-section through the area that a water-charged section exists in the formation directly beneath the gas-charged sweet spot, Fig. 2. The most intense portion of the velocity anomaly within the targeted reservoir falls nearly 2,000 m/s below the normal regional velocity / depth gradient at the depth of the target, Fig. 1; thus, the rock / fluid volume of interest is characterized by intensely slow velocities, i.e., the interval has a very high probability of being gas-charged.²

Fig. 2. Anomalous velocity cross-section through the area shown in map view in Fig. 1. Note relief on regional velocity inversion surface (or regional pressure surface boundary). Most important, the cross-section shows water-rich domains above and below gas-charged rock / fluid system.

The anomalous velocity volume also shows that there is a very significant topographic high on the regional velocity inversion surface, Fig. 2. This topographic high represents a gas chimney where gas is moving up into the stratigraphic section along a significant migration conduit. At the level of the targeted reservoir interval, there are intense, non-oriented trace-to-trace discontinuities, which, in this case, are interpreted to be fracture swarms. It is important to note that there is an impressive overlap between the anomalous velocities, Fig. 1, and the trace-to-trace discontinuity anomalies, Fig. 3. This overlap likely represents an area or volume characterized by gas-filled (as indicated by the anomalous velocities) fracture swarms (as indicated by the trace-to-trace discontinuities).

Fig. 3. Gas distribution and microfracture swarm distribution. Superposed anomalous velocity and trace-to-trace discontinuity maps at level of targeted reservoir interval. Overlap of intensely slow velocities with intense discontinuities is likely attributable to a gas-charged fracture swarm.

The overlap of these key elements strongly suggests that: 1) significant amounts of gas are present at the targeted-interval depth, e.g., tight gas sand; 2) enhanced porosity / permeability is present in the form of fracture swarms; and 3) gas content of fluid within the reservoir interval is relatively high in the area of interest. The "spider" diagram, i.e., inventory of all linear faults with lengths greater than 1,000 ft, constructed for the targeted reservoir interval, shows that there are numerous linear faults and fault intersections in the area. This finding supports the earlier conclusion that fracture swarms characterize the area, particularly in the vicinity of the most intense velocity anomaly.

When superimposed, all of these key interpretive elements, see table, overlap significantly within the targeted reservoir interval in the area of interest, Fig. 4. Within the interpretive element overlap area, the probability of finding commercial anomalously pressured gas assets in zones of enhanced porosity and permeability (production sweet spots) is extremely high.

Fig. 4. Superimposed positive critical exploration elements: gas migration conduits, gas distribution, reservoir gas content, microfracture swarm distribution and linear fault orientation. Overlap of these elements is interpreted as a gas-charged domain with intense microfracture swarms. Well symbol shows location of a successful gas well producing from low-permeability, "tight" gas sandstones.

Key interpretive elements for sweet-spot delineation

• Gas distribution (GD)
• Gas migration conduits (GMC)
• Microfracture swarm distribution (MSD)
• Reservoir gas content (RGC)
• Linear fault orientation (LFO)
• Reservoir characterization at-tributes (RCA)

The final success of a particular well in this area, however, is predicated on avoiding water-dominated fluid systems that occur just below and adjacent to the gas-charged sweet spot, Fig. 2, and on designing a careful drilling / completion program based on well-conceived mud and frac programs that avoid fluid incompatibilities and formation damage. In this case study, drilling occurred within the interpretive element overlap zone (Ven Diagram). Water-dominated fluid systems both below and adjacent to the sweet spot were avoided; and the drilling / stimulation program minimized formation / fluid damage, all of which resulted in a successful well.

This case study is a neat example of the type of detailed information required to intelligently develop anomalously pressured gas assets. First, the gas accumulation must be identified and delineated; next, production sweet spots must be described in detail and a careful drilling / completion program must be designed based on this description; and, finally, problems such as excessive water production, formation damage and fluid incompatibilities must be avoided. If any of these steps are omitted, exploration uncertainty and subsequent risk increase dramatically. The IDT technology allows operators to accomplish these steps and their ultimate goal – a successful exploration and exploitation program for anomalously pressured gas resources.

Acknowledgment

Special acknowledgment goes to GRI (GTI) and DOE for providing financial support to allow the development of concepts for exploring anomalously pressured gas accumulations in the Rocky Mountain Laramide basins. Dynamic Graphic Earthvision^TM software was used in the geospatial modeling and visualizations presented in this article. ProMax seismic processing and Landmark seismic interpretation software also were used in this project.

Literature Cited

¹ Surdam, R. C., J. Robinson, Z. S. Jiao and N. K. Boyd, "Delineation of Jonah field using seismic and sonic velocity interpretations," in D. Anderson, J. Robinson, J. Estes-Jackson and E. Coalson, Eds., Gas in the Rockies: Rocky Mountain Association of Geologists, 2001, pp. 189 – 209.

² Surdam, R. C., "A new paradigm for gas exploration in anomalously pressured ‘tight gas sands’ in the Rocky Mountain Laramide basins," in R. C. Surdam ed., Seals, traps and the petroleum system: American Association of Petroleum Geologists, Tulsa, Oklahoma, Memoir 67, 1997, pp. 283 – 298.

The author
	Ronald C. Surdam, president, Innovative Discoveries Technologies, LLC., Laramie, Wyoming, earned AB and PhD degrees from the University of California in 1961 and 1967. From 1966 to 1998, he served as assistant professor and professor of geology at the University of Wyoming. From 1993 to1998, he was director of the Institute for Energy Research, and from 1997 to 1998, the Enhanced Oil Recovery Institute. Before assuming his present position in 2000, he was a professor emeritus and consultant to the University of Wyoming. Dr. Surdam has pursued numerous professional activities, including: GSA, AAPG, SEPM, Sigma Xi, Wyoming Geological Assoc. and SPE. He has received several professional awards and been a distinguished lecturer for several organizations; and he has contributed more than 180 publications.

	Part 1: Exploration Methods
	Part 2: North America Outlook
	Part 3: HP/HT Drilling and Completions
	Part 4: Tight Formation Stimulation
	Part 5: Sour Gas Handling
	Part 7: Gathering and Compression
	Part 8: Monetizing Stranded Gas