A common question when dealing with tight matrix naturally fracturedreservoirs is - How can we determine if the matrix is contributing tohydrocarbon production? One source of information for helping to answer thisquestion is provided by matrix and fracture permeabilities determined with thePressure-Decay Profile Permeameter (PDPP). The useful range of the tool is from0.001 to 20,0000md.
These permeabilities are corrected for overburden conditions and are utilizedto numerically simulate the interaction between matrix and fractures in a core.The procedure involves the following steps:
From PDPP measurements create iso-permeability maps of the fracturecore.
Discretize the permeabilities using a micro-gridover the core area.
Select a ‘production’ cell in the fractures and various observation cellsin the matrix.
Simulate the core using reservoir pressure, temperature and fluidproperties.
The results indicate if the very low matrix permeability contributes tohydrocarbon production in the naturally fractured reservoir where the core wascut.
There are many naturally fractured reservoir around the world in all kinds oflithologies throughout the various geologic periods. This type of reservoirscontains significant amounts of oil and gas resources. They presents botheconomic opportunities and technical challenges. Inorder to properly exploitnaturally fractured reservoirs, engineers and geologists have developedspecialized techniques and tools to help in their evaluation.
The overview of the characteristics of naturally fractured reservoirs andtechniques for their analysis have been the subject of various textbooks1–5. In a paper on recent advances in the study of naturallyfractured reservoir, Aguilera6 summarizes the sources of informationavailable to evaluate naturally fractured reservoirs.
In a very simple model, a naturally fractured reservoirs consists of matrixrock with high storage and low permeability. These matrix rock is generally notcapable to sustain commercial production without natural fractures. Thefractures have very low storage capacity but high permeability. Fluid in thematrix can bleed off into the fracture network and then be transported throughthe ractures to the wellbore. An often asked question is whether fluid storedin the tight matrix actually contribute to production. A new technique forpermeabilitymeasurements in cores coupled with numerical microsimulation offerssome answers to this question.
This could be combined with a laboratory technique that allows to measureseveral hundred pressure readings in a short span of time to study the responseof cores to pressure disturbances. This laboratory technique allows quick andaccurate determination of matrix and fracture properties as reported by Kamathet al. 7
A carefully handled core from a naturally fractured reservoir provides valuableinformation on reservoir properties, including whole core porosity andpermeability. From well logs we can estimate porosity in the matrix, andractures. Pressure transient analysis can also add information includingfracture porosity and permeability. All these estimates are related to the bulkproperties of the system. With a new permeability measuring equipment, thepressure-decay profile Permeameter8–10, detailed description ofpermeability distribution can be obtained.