SIPES Monthly Luncheon
Friday, September 18, 2014
Houston Petroleum Club
Social 11:15 am Meeting 12:00 pm
Speaker: Dr. Ron Nelson
Additional information can be found on sipeshouston.org.
The Importance of Natural Fractures in a Variety of Unconventional Reservoirs, Focusing on the Quantitative Assessment of Natural Fracture Intensity and Mechanical Properties
Typically, the effect of natural fractures is ignored in development programs in mudrock and other unconventional reservoirs. The rationale is that we can create all the fractures we need for production through our stimulation and completion processes. Indeed in many of the potential drill locations and well paths in these reservoirs, natural fracture intensity is low and their effects are negligible. However, in some of the drilling and completion programs we encounter a number of wells with natural pre-stimulation hydrocarbon flow. Also, some wells produce at rates and volumes greater than normal, experience rapid water influx, or notice stimulation stages that are anomalously positive or negative. In many cases, these results can be explained by the origin, distribution and flow characteristics of local natural fractures. These natural fractures derive from a variety of tectonic and regional forces and their relative distribution can be linked to local tectonic strain and the mechanical property distribution within individual reservoirs. One misconception commonly held in the industry is that mudrock and other unconventional reservoirs are inherently similar in properties and in the procedures needed to effectively develop them. In addition, development procedures are often deemed be the same for all locations within the development area. With these concepts in mind, the conclusion is made that what we learn for effectively developing one of these reservoirs will be the same in all of them. Unfortunately, this is not the case. These reservoirs vary significantly in their properties; such as structural inhomogeneity of fractures, compositional make-up, petrophysical properties, depositional fabric, and three-dimensional mechanical properties.
This presentation will focus particularly on quantitative aspects of natural fracture and mechanical property variability between unconventional mudrock reservoirs. In addition, examples of the effect of these property variations on production and stimulation will be discussed. Analyses presented are based on quantitative studies of 11 unconventional reservoirs from over 230 vertical wells and cores. These include Upper, Middle and Lower Bakken (Dev-Cm), Three Forks (Dev), Niobrara (K), Eagle Ford (K), Pearsall (K), Woodford (Cm), Poland Sh (Ord-Sil), and 2 (Jr) reservoir units in the Middle East.
Results show that average unit natural fracture intensity (#/ft intercept rate as measured in vertical cores) varies from 0.310 to 43.045 between the reservoirs studied. Mechanical properties of dynamically derived average unit Rigidity Modulus or Shear Modulus (G in GPa) for the same reservoirs vary from 7.77 to 19.79. In general, higher Rigidity indicates higher rock stiffness generally leading to higher natural fracture intensity for fractures generated by far field stress states, like folding-related fractures and regional fractures, Nelson (2001). The local relation between relatively high Rigidity and corresponding relatively high natural fracture intensity will be shown in several unconventionals.
Variation in natural fracture intensity and mechanical properties must be taken into account in creating Static Conceptual Fracture Models for reservoir simulation and in planning drilling and completion programs in unconventional reservoirs. It is recommended that quantitative natural fracture studies be conducted in all major unconventional reservoir developments in order to effectively predict reservoir behavior locally within the play. This includes determination of the dominant structural style(s) of the particular reservoir trend. For example, the structural environment of the Woodford previous compressional terrain in Oklahoma is inherently different in its’ deformation than that of the active extensional terrain of the Gulf Coast in much of the Eagle Ford Trend. These differences by unconventional reservoir will also be highlighted in the presentation.
Ronald A. Nelson has worked with fractured reservoirs for over 39 years with Amoco and BP Amoco and since 2001 with his own company, Broken N Consulting, Inc. He has taught numerous courses on fractured reservoirs for the AAPG, SEG, Nautilus, NExT and individual companies and has authored over 100 publications on structural geology and fractured reservoirs, including the 1985 and 2001 editions of a textbook entitled “Geologic Analysis of Naturally Fractured Reservoirs”. He has lectured on the subjects of structural geology and fractured reservoirs to Geological Societies, Universities, and National Oil Companies in over 20 countries and has been twice an AAPG Distinguished Lecturer and an SPE Distinguished Author.