A case study has been conducted on 90 gas wells in West Texas and 3 gas wells in northern Alberta to improve fracture, conductivity and post fracture productivity. This study shows the effectiveness of various treatment designs and breaker systems. These wells have shown improved cleanup with higher fluid recoveries and higher post frac flow rates in comparison to surrounding wells.
Laboratory studies have shown that insitu fracture conductivities are considerably lower than what they were estimated in the past. This is due to reduced proppant permeability due to crushing and embedment as well as damage to the proppant pack due to the presence of the polymer gelling agents. New testing procedures have quantified the magnitude of this damage and the concentration of breaker necessary to minimize this damage. When the breaker concentration level is increased to the level necessary to degrade the concentrated polymer, the fluid rheology is compromised and the placement of the proppant could be jeopardized. Encapsulation of the breaker has allowed the placement of elevated concentrations of breaker within the proppant pack and a controlled release after the proppant is placed.
Productivity of a propped hydraulically fractured well depends on the on many factors including the reservoir permeability, fracture length and width, and the permeability of the proppant. The relationship between these parameters has been combined in the relationship of dimensionless fracture conductivity.
Equations (available in full paper)
To optimize the production increase expected from a hydraulic fracture treatment, a Crd of at least 10 is suggested. To achieve this, the conductivity (k1w) portion of the above expression must maximized increasing the dimensionless fracture conductivity by increasing fracture width through addition of high proppant concentrations is initially beneficial, but eventually limited. Obtaining propped fracture width in excess of 6 mm is difficult if not impossible to achieve under most fracturing conditions. Therefore great care mus tbe taken to insure that the proppant permeabliity remains as high as possible.
For the last twenty years the conductivity of the proppant pack that results from hydraulic fracturing has been considerably over estimated. Proppant permeability values were determined by performing short term tests using equipment first described by Cooke1. Proppant is placed in a cell capable of directly applying stress and temperature to simulate downhole conditions. The permeability of the pack is measured by flowing fluid through the cell and measuring the pressure drop. This methodology was adapted by the industry and gained wide industry acceptance when adopted by the API.2 Many authors have recognized the need for more thorough testing to estimate the insitu proppant pack permeabilities. 345 Studies were conducted to measure the long term effects on the proppant pack. The results of these studies showed that proppant permeability is significantly reduced over a period of 24 hours as the proppant grains realign and then starts to stabilize for the next 24 to 72 hours allowing the testing to stop after 100 hours.