Recovery Factors And Reserves In Naturally Fractured Reservoirs

Abstract

Introduction Conventional reservoir engineering techniques and naturally fractured reservoirs do not mix well. The use of conventional techniques has led to underestimating or overestimating recoveries and reserves in many naturally fractured reservoirs worldwide. This paper is a follow-up to a previous article dealing with advances in the study of naturally fractured reservoirs(1). In that article I concentrated on types of fractures, how to intersect them, and on key items associated with data acquisition. In this paper I provide general information dealing with recovery estimates and reserves in naturally fractured reservoirs. The paper is intended for the general interest reader who is not a specialist in the field. Recovery Not all fractured reservoirs are the same. So talking about fractured reservoirs in general is not good enough. My recommendation is to initially classify the reservoir according to (1) geologic, (2) pore system, (3) hydrocarbon storage, and (4) matrix/fracture interaction points of view. Geologic Classification From a geologic point of view the fractures can be classified as being tectonic (fold and/or fault related), regional, contractional (diagenetic), and surface related(1–3). Historically most hydrocarbon production has been obtained from tectonic fractures, followed by regional fractures and followed by contractional fractures. In general, surface related fractures are not important from the point of view of hydrocarbon production. When classifying the fractures determine fracture dip and strike. Pore Classification It is possible to make preliminary estimates of productive characteristics of common reservoir porosity types following a classification proposed by Coalson et al(9). In this classification porosity classes are defined first by the geometry of the pores, and second by pore size. Included in. the geometry are the following pore categories: Intergranular, Intercrystalline, vuggy, and fracture. The combination of any of them can give origin to dual and even multiporosity behavior. Included in the pore size are megaporosity, macroporosity, mesoporosity, and microporosity. Table 1 shows typical petrophysical parameters for this classification adapted from a combination of Coalson et al. (9), and White(10). Included in this Table are the geometry, pore size, pore throat radius at 35% mercury saturation (Winland R35 values), permeability to air, immobile water saturation and typical capillary pressure curves coded A through D. These capillary pressures are shown on Figure 1. The aperture of the fractures and vugs deserve further discussion. From laboratory work and experience it has been found that nut shells and plastic materials can stop circulation losses in fractures with apertures as large as 5,000 microns. If in a given naturally fractured reservoir these materials cannot stop circulation losses the conclusion is reached that the apertures are bigger than 5,000 microns. In fact, secondary porosity apertures can actually reach cavern-size in some instances. Storage Classification From a storage point of view the fractures can be classified(3) as being of Type A, B or C. Many reservoirs that would otherwise be non-productive are commercial thanks to the presence of natural fractures,(8) In reservoirs of Type A the bulk of the hydrocarbon storage is in the matrix porosity and a small amount of storage is in the fractures.

The Lancaster Field: progress in opening the UK's fractured basement play

“…Note that veins are mineral-filled fractures and therefore do not contribute to reservoir porosity or permeability. The range of estimated fracture apertures is consistent with extremely high fracture permeability (K air ) and zero immobile water saturation (S w ) (Aguilera 1999). which compares favourably with the previously reported wireline derived value of 4.7% (Trice 2014).…”

“…As demonstrated in Figure 7, the potential distribution of apertures within the Lancaster reservoir is from 20 mm to 2 m. This is a significant distribution, as it implies a highly permeable hydrodynamic fracture network and also indicates that irreducible water will not be present: therefore, no fluid transition zone is to be expected (Aguilera 1999). Given the uncertainties present in estimating effective fracture apertures from image log data, aperture-derived fracture porosities are considered to be associated with a too large uncertainty and, as a consequence, fracture porosity is estimated from bulk porosity methods.…”

Basement exploration, West of Shetlands: progress in opening a new play on the UKCS

“…This fact, coupled with disparities in the measured volume for formation between resistivity tools and porosity tools, further compounds the problem, also exacerbated by the heterogeneous and commonly anisotropic poro-perm systems present in Type 1 fractured reservoirs. While it can be inferred that the irreducible water saturation within a fracture measured at the core plug scale is of order 0 -20% (Aguilera 1999;Ngoc et al 2007), the effective porosity system at Lancaster is not, and cannot be, represented by core or coreplug-scale measurements as the flowing fractures as detected by image log and PLT comparison cross-cut the borehole as discrete planes. Such fractures are commonly associated with apertures of sufficient magnitude to cause discernible responses on conventional and high-resolution wireline logs; with apertures estimated to be commonly in excess of 2 cm, such fractures are anticipated to be associated with zero irreducible water (Aguilera 1999).…”

“…While it can be inferred that the irreducible water saturation within a fracture measured at the core plug scale is of order 0 -20% (Aguilera 1999;Ngoc et al 2007), the effective porosity system at Lancaster is not, and cannot be, represented by core or coreplug-scale measurements as the flowing fractures as detected by image log and PLT comparison cross-cut the borehole as discrete planes. Such fractures are commonly associated with apertures of sufficient magnitude to cause discernible responses on conventional and high-resolution wireline logs; with apertures estimated to be commonly in excess of 2 cm, such fractures are anticipated to be associated with zero irreducible water (Aguilera 1999). Despite the inability to measure S W it is used in the volumetric analysis; however, it is applied primarily to provide an estimate of perched water distribution.…”

Advances in the study of naturally fractured hydrocarbon reservoirs: a broad integrated interdisciplinary applied topic

“…Usually production from non-fractured reservoirs can be predicted accurately to within an acceptable margin. Historically however there has been a much larger range in variability of recovery achieved from fractured reservoirs (Aguilera 1999). This has traditionally made it difficult to reliably predict recovery from fractured reservoirs, leading to a large degree of uncertainty in reservoir development.…”