Abstract
Economical production from low-permeability oil-saturated reservoirs has always been a challenge in a basin known for its mature assets. M2 limestone is a new challenge. To characterize, it was necessary to use the methodology based on shale plays, integrating information from different logs using a proprietary evaluation method. Applying pillar fracturing, creating stable voids between pillars, and hence, infinite-conductivity channels in geomechanically competent candidates resulted in economical production and proved reserves from a low-permeability calcareous shale.
Geomechanics, mineralogy, and saturated intervals were addressed by using a combination of rock mechanical properties and mineralogy, carbon/oxygen logs, and X-ray diffraction (XRD) on drilling cuttings. Once the prospective zones in the M2 limestone intervals were selected, a conventional fracturing treatment was designed using a 3-D gridded simulator. The candidate well was evaluated for pillar fracturing by using results from geomechanics and the conventional fracture application. A pumping schedule that included pillar volume, spacer, and tail in stages was then designed. Results from the fracture simulator were loaded in a numerical reservoir simulator, and different development scenarios were evaluated.
M2 limestone has shown production potential near areas where volcanic intrusion is present, or indicated hydrocarbon potential by oil shows observed on cuttings and high-gas readings during drilling. The data used for this project was collected during conventional reservoir development but had never been evaluated using an unconventional reservoir approach. XRD analysis and acid solubility tests confirmed that the reservoir does not contain a high-carbonate content nor acid solubility. Diagnostic Fracture Injection Test (DFIT) and minifrac analysis helped to define the size and fracturing technique to be used. Results from this work provided a better understanding of the reservoir; a development plan is needed to improve the investment return for this type of project. Geomechanical evaluation is fundamental to the application and design of pillar fracturing. This fracturing technique was selected because it used 43% less proppant than a conventional job, reduced risk of screen out, and provided higher productivity over a conventional fracturing job. This is the first time that pillar fracturing has been applied in this Ecuadorian reservoir. The production outcome proved reserves of 32°API oil and resulted in the largest fracturing job in Ecuador. Different development scenarios are proposed based on the results from this well.
A complete workflow to characterize, design a hydraulic fracture job using proprietary geomechanical candidate selection criteria, and develop an unconventional calcareous shale is presented. The available data are the same as in a conventional reservoir, whereas the evaluation technique, as well as fracture design, is customized to this type of reservoir to attain economical production.
An evaluation method of volume fracturing effects for vertical wells in low permeability reservoirs
