Information on fractured reservoirs is often controversial. Engineers see lost circulation, negative skin and fracture well test signatures. Geologists see only matrix properties in their cores. Geologists see fractures but engineers see only radial flow on their well tests. In many cases, the two lines of information concur and the evidence is uncontroversial. In other cases the information is not so clear. Engineering data is notoriously non-unique and because carbonate reservoirs have such high heterogeneity—over 30 possible forms of porosity—and many ways this can be connected (or not!) this is a real challenge. What is seen by geologists in small cores may not be seen in larger well tests. Alternatively what is ‘seen’ in the well tests may bear no link to the observed rocks. It is in these circumstances that the two specialists need to come together and understand each others points of view and the limitations of each other’s data. This requires specialist knowledge with geoengineering insights to try and reach unification of geological and engineering models. All models are wrong—but the one both disciplines agree with is probably useful.

It is quite common for reservoir engineers to adjust the geological modelling without recoursing to the geologists by multiplying the porosity, the permeability, the anisotropy (kv/kh), the relative permeabilities, the well factors and many other parameters within their numerical world. Sometimes these factors can be large and global and probably outside the limits of the geological reality. Of course it is not easy to go back and make these adjustments in a close cooperative environment for all sorts of reasons—logistical, technical, management, contractual to name a few. Rarely are these adjustments discussed and certainly there are very few published examples where the loop has been closed. This talk will attempt to illustrate where and how multipliers are applied, what might be the reasons and how the workflows could be streamlined to make closing-the-loop a routine process rather than an occasional occurrence.

Challenges for global gas shale production include infrastructural and geological with the Marcellus providing an analogue for both. (Infrastructural) The production from the Marcellus gas shale presents unique challenges that include issues associated with leasing, geology, landowners, virtually no deep disposal wells, state governments without a severance tax, several river basin commissions, an infrastructure designed for shallow gas production, an emotional group of environmentalists, and one state that has yet to permit horizontal well stimulation. This combination of challenges makes for a very interesting set of lessons that operators will face elsewhere in the world when attempting to play gas shales. (Geological) The Appalachian Basin is characterized by 2nd order depositional sequences (approximately 10’s of million years duration) that make up thousands of feet of strata in this basin, 3rd order sequences (1-10 million years) with up to several hundred feet of strata, and parasequences, that comprise tens of feet of strata. Middle Devonian Marcellus Formation encompasses two third order transgressive-regressive (T-R) sequences, MSS1 and MSS2, in ascending order. Compositional elements of the Marcellus Formation crucial to the successful development of this emerging shale gas play, including quartz, clay, carbonate, pyrite, and organic carbon, vary predictably within the proposed sequence stratigraphic framework. Tops of the parasequences commonly contain a calcareous interval, commonly containing shell debris, overlain by a sharp transition into the high TOC mudrocks of the next overlying parasequence. Thickness trends of Marcellus T-R sequences and lithostratigraphic units reflect the interplay of Acadian thrust-load-induced subsidence, short-term base-level fluctuations, and recurrent basement structures. Rapid thickening of both T-R sequences, especially MSS2, toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the Acadian orogeny. However, local variations in T-R sequence thickness in the western, more distal, area of the basin may reflect the reactivation of inherited Eocambrian basement structures to form a carbonate bank.

Assets within the Appalachian Basin range from conventional clastic and carbonate reservoirs to source rocks of Devonian black shale and Pennsylvanian coal, all of which are fractured. These fractures range from coal cleat and cracks around kerogen flakes to natural hydraulic fractures, tensile joints in stiff beds, and late-stage cross joints. With some exceptions this broad range of fracture types propagated with the help of pressure generation accompanying the positive ΔV reaction during maturation of hydrocarbons. Before and during maturation fracture orientation in the Appalachian foreland was controlled by an evolving tectonic stress that reflects three important details of Acadian-Alleghanian orogenesis in the Appalachian hinterland. First, pre-maturation, forebulge-related tensile joints in distal portions of the Acadian Catskill Delta complex reflect initial loading of Laurentia (i.e., North America) by Gondwana (i.e., Africa) at the New York promontory. The earliest syn-maturation fractures are microcracks around kerogen flakes in black shale. Maturation-related pressure was enhanced by compaction disequilibrium. Maturation continued to elevate pressure within Devonian black shales to the point that macroscopic natural hydraulic fractures (NHF) developed within the source rocks. The orientation of NHF in black shale and early coal cleat in the foreland reflects a basin-wide stress field arising from the oblique convergence of Gondwana and Laurentia, the second detail of Acadian-Alleghanian orogenesis. This basin-wide joint system supports the emerging view that dextral transpression controlled the kinematics in the mountain belt to a greater extent than previously recognized. Further burial led to the development of a complete fracture network in siltstones and gray shale of the basin. This later system of fracturing evolved in Alleghanian stress fields arising when transpressional tectonics within crystalline basement, the third detail, drove the classic detachment sheets of the Valley and Ridge and Appalachian Plateau.

Production from the Marcellus gas shale generated international interest when methane accumulated in the surface housing of a water well pump and exploded. The Pennsylvania Department of Environmental Protection (PA-DEP) immediately investigated and determined the cement had insufficiently isolated shallow methane-bearing sands (not the Marcellus gas shale) and methane from these sands was leaking into ground water. The media immediately seized upon the story and painted a picture of an industry unable to manage risk. The reputation of gas shale was further darkened by a Hollywood polemic called, Gasland, a documentary based loosely on facts. Later there were two highly publicized blowouts from Marcellus wells and some surface spills that added strength to those who argued against industry. The biggest public fear was the frack fluid could, somehow, flow uphill more than 2000 meters to contaminate groundwater. Of course, there are a number of physical laws such as the law of gravity and the law of buoyancy that prevent this from happening. Since the initial hype by the media, studies by both the US Environmental Protection Agency (EPA) and PA-DEP have shown beyond a shadow of a doubt that no frack fluids have contaminated groundwater in the vicinity of the methane leaking from casing. Microseismic surveys have since shown that fracture stimulations travel laterally as much 300 m but are generally restricted in vertical growth to 100 m. The focus of the fracking debate has since shifted to overlap with the climate debate. The debate was further sharpened by academic studies claiming such a high rate of methane leakage during completion that the effect on global warming would be substantial even though burning methane releases about half the CO2 relative to that released by coal on a BTU basis.

Craquelure, the fine pattern of cracks found in old paintings, presents a rare opportunity to reach beyond the physical sciences for help in understanding a geological process as inscrutable as the development of joints in a fractured reservoir. Masterpieces in the Louvre (Paris, France) and other national galleries, like bedded sedimentary rocks, are jointed (i.e., cracked) composite materials with welded contacts between substrate and joint-bearing medium. The analogy goes even further because differences in properties between the substrate and the joint-bearing medium greatly influence a number of characteristics of joint growth patterns including fractal properties (Duccio, 1300), propagation direction (Duccio, 1311), spacing vs. bed thickness (Francesca, 1455), orientation (Master of St. Giles, 1500), abutting relationships (Clouet, 1530), degree of systematic development (Rembrandt, 1660) and lack of systematic development (Chardin, 1736). In particular, the degree to which the joint-bearing medium is subject to ‘tectonic’ stress predetermines the extent to which a number of these joint patterns develop (Hals, 1647). Regardless of the particulars, it is clear that the masterpieces and sedimentary rocks even share two common loading configurations: the joint-normal load (canvas support) and the thermoelastic load (wood support). When ‘tectonic’ stresses are not present mudcracks (Fouquet, 1455), columnar joints, and spiral joints (David, 1791) propagate under thermoelastic loading. In the masterpieces, ‘tectonic’ stresses arise from the way the substrate was stretched, dried, or woven and these stresses are responsible for controlling the orientation of systematic joints again under joint-normal loading (David, 1813). Systematic joints may fan or curve if the substrate provides local stress concentrations (da Vinci, 1503). Preparation may also cause the substrate to deform in oroclinal-like bends as indicated by concomitant jointing in the host medium (Cassatt, 1893). Finally, cross joints are less systematic and, hence, the role of tectonic stress is less clear (Goya, 1798).