Recognizing naturally fractured reservoirs

Fractures are the most abundant visible structural features in the Earth’s upper crust. They are apparent at most rock ridges, and it is likely that most reservoirs contain some natural fractures. Naturally fractured reservoirs are elusive systems to characterize and difficult to engineer and predict. It is important to establish some basic criteria for recognizing when fractures are an important element in reservoir performance and to recognize the nature and performance characteristics of a naturally fractured reservoir.

How naturally fractured reservoirs are formed

Unlike induced fractures, natural fractures are caused by stress in the formation usually from tectonic forces such as folds and faults. Natural fractures are more common in carbonate rocks. Fractures occur in preferential directions, determined by the direction of regional stress. This is usually parallel to the direction of nearby faults or folds, but in the case of faults, they may be perpendicular to the fault or there may be two orthogonal directions.

Open and healed fractures

A fracture is often a high permeability path in a low permeability rock, or it may be filled with a cementing material, such as calcite, leaving the fracture with no permeability. It is important to distinguish between open and healed fractures. The total volume of fractures is often small compared to the total pore volume of the reservoir.

Size

Naturally fractured reservoirs are observed across a vast range of scale from microcracks to mile long features. The vertical extent of fractures is often controlled by thin layers of plastic material, such as shale beds or laminations, or by weak layers of rock, such as stylolites in carbonate sequences. The width of these beds may be too small to be seen on logs, so fractures may seem to start and stop.

Types

Naturally fractured reservoirs can be open, permeable pathways, or they can be permeability baffles resulting from the presences if secondary mineralization or other fine-grained material filling the gaps. Most natural fractures are more or less vertical. Horizontal fracture may exist for a short distance, propped open by bridging of the irregular surfaces. Most horizontal fractures, however, are sealed by overburden pressure. Both horizontal and semi-vertical fractures can be detected by various logging tools.

Classification

Naturally fractured reservoirs have been classified according to the relative contribution of the matrix and fractures to the total fluid production. The following table is modified form Nelson (2001).

Classification of naturally fractured reservoirs (NFR), modified from Nelson (2001)
Types of fracture reservoirs
NFR type	Definition	Examples
Type 1	Fractures provide essential porosity and permeability.	Amal, Libya Edison, California Basement fields, Kansas
Type 2	Fractures provide essential permeability	Agha Jari, Iran Haft Kel, Iran Sooner trend, Oklahoma Spraberry trend area, Texas
Type 3	Fractures provide a permeability assistance	Kirkuk, Iraq Dukhan, Qatar Cottonwood Creek, Wyoming Lacq, France

Productivity heterogeneity

Well production heterogeneity is a characteristic of most naturally fractured reservoirs. Although various geologic processes other than fractures can lead to field wide heterogeneity, such as thin, discontinuous high-permeability strata or variable development of interconnected vugs. Heterogeneity can be used to help recognize the importance of fractures in a reservoir and production statistic for wells can provide a quick-look method to recognize heterogeneity.

Indications of fractures in a reservoir

Indications of fractures in a reservoir
Production/flow-related evidence
Observation	Possible explanation	Precision
Isolated intervals of high productivity on PLT that do not correspond with good reservoir rock	Fractures enhance permeability	Specific depth in well
PI of well exceeds expectations	Fractures enhance permeability	Reservoir
khwell/khcore  >>1	Fractures enhance permeability (core analyses should exclude samples with fractures)	Well or reservoir
Low porosity, low resistivity	Low fracture porosity, but good conductivity due to fractures filled by drilling fluid	Specific depth in well
Significant unexpected gas show or "kick," especially in low­ permeability rock	Light hydrocarbons in fracture	Specific depth in well
Dual-ø  behavior in well test	Fracture drainage, followed by matrix drainage; the absence of this effect is not evidence against the presence of fractures	Reservoir
Highly productive well shows rapid decline in productivity	Fractures intersected by well are of limited extent, and drain quickly, so system reduces to drainage from matrix	Reservoir
Rapid rise in gas/oil ratio to water/oil ratio (GOR/WOR), then stabilization at a level >> solution GOR	Dual system behavior; fractures drain first, then gas and water enter, then matrix feeds fractures, and a new steady state is achieved	Reservoir
Water or gas coning	High vertical conductivity of fr ctures allows fluids to be rapidly drawn across stratigraphic layers to low-pressure area near producing well	Reservoir, specific depth
High well-to-well productivity variability	Fracture intersections can result in great variation in well performance	Reservoir
Well-to-well! hydraulic communication	Movement of fluids between wells along aligned fractures	Reservoir
Rapid breakthrough of injected fluids along a consistent trend	Movement of fluids between wells along aligned fractures	Reservoir
High vertical connectivity	Fractures cross-cut low-k layers	Reservoir
Strong anisotropy in multiwell pulse test	Rapid hydraulic conductivity along oriented fracture set	Reservoir
Drilling evidence
Observation	Possible Explanation	Precision
Drilling breaks	Rapid drilling progress along fractures, especially through intervals where rapid drilling is unexpected	Specific depth in well
Bit drop	Open fracture, usually enhanced by solution enlargement to create a large open cavity	Specific depth in well
Bit chatter	Bit-fracture interaction	Specific depth in well
Abrupt loss of circulating fluids	Mud drained into extensive open fracture system. Losses can damage reservoir	Specific depth in well
Geological evidence
Observation	Possible Explanation	Precision
Open natural fractures evident in core or image logs	Fractures in the reservoir are open	Specific depth in well
Spikes in Stoneley wave reflectivity log	Fractures are open in the reservoir	Specific depth in well
Hard rock in the reservoir	Brittle rock has a tendency to respond to strain by creation of open fractures	Reservoir
Same reservoir productive from fractures elsewhere	Subsurface fractures are a characteristic of this formation	Reservoir
Geophysical indications of fractures
Observation	Possible Explanation	Precision
Shear wave polarization in seismic, vertical seismic profile (VSP), full-wave sonic	Birefringence due to open fractures. Effect may also be caused by stress-induced birefringence (fractures not necessary)	Portion of reservoir
Shear and P-wave velocity anisotropy in VSP or surface seismic	Seismic wave velocity varies as a function of raypath orientation relative to fracture set orientation	Portion of reservoir
Anisotropic seismic attenuation	Seismic attenuation vades as a function of raypath orientation relative to fracture set orientation	Portion of reservoir

Noteworthy papers in OnePetro

Aguilera, R. 1983. Exploring For Naturally Fractured Reservoirs. Society of Petrophysicists and Well-Log Analysts. https://www.onepetro.org/conference-paper/SPWLA-1983-C.

Tiab, D., Restrepo, D. P., & Igbokoyi, A. O. 2006. Fracture Porosity of Naturally Fractured Reservoirs. Society of Petroleum Engineers. doi:10.2118/104056-MS. http://dx.doi.org/10.2118/104056-MS

Trask, P. D., & Patnode, H. W. 1936. Means of Recognizing Source Beds. American Petroleum Institute. https://www.onepetro.org/conference-paper/API-36-368.

External links

Xiong, Hongjie. 2017. "Optimizing Cluster or Fracture Spacing: An Overview." The Way Ahead. Society of Petroleum Engineers. https://www.spe.org/en/twa/twa-article-detail/?art=3007

Nelson, R.A. 2001. Geologic Analysis of Naturally Fractured Reservoirs. Gulf Publishing. http://www.worldcat.org/oclc/45755956.

See also

Fluid_flow_in_naturally_fractured_reservoirs

Fracture_diagnostic_techniques