Fracture conductivity values observed by well testing following hydraulic fracture stimulation treatments are often much lower than the values obtained by laboratory measurements for most proppants. In addition, many stimulated wells show loss of fracture conductivity with time, leading to reduced productivity and lost revenue.
This rapid loss in fracture conductivity has been attributed to frac gel damage, embedment, proppant crushing, and fines invasion. All these damage mechanisms have been well studied in the laboratory, resulting in materials and methods employed to minimize their effect on conductivity. While these methods have improved productivity, stimulated wells rarely achieve the theoretical conductivity expected from a given proppant. In addition, fracture conductivity often declines continuously, suggesting that there are other factors that influence long-term fracture conductivity.
Recently it was suggested that a freshly placed proppant bed in a hot formation could undergo a geochemical process called diagenesis. This process involves the dissolution of some proppant minerals and subsequent reaction with minerals that are present in the connate formation water. These subsequent reactions lead to the formation of a host of many clays and clay-like crystals in addition to amorphous, porosity-filling materials that progressively damage the conductivity of propped fractures.
This paper presents (1) results of a laboratory study aimed at quantifying this damage mechanism, (2) the development of multiple strategies to minimize this damage mechanism, and (3) field case studies analyzing production data to support two of these laboratory-indicated strategies.
Findings reported in this paper should have a significant impact on how proppants are selected for various fracturing applications. To achieve maximum fracture conductivity and sustained production, it is no longer acceptable to simply select the proppants, based on API conductivity, cost, and availability, but planners must also consider (1) potential in situ reactions that may shorten proppant life or (2) proppant treatments that can be applied to minimize the impact of these reactions.
This section presents and discusses the damage mechanisms that decrease formation conductivity, and consequently, may decrease production.
Fracture Conductivity. A fracture generated during a hydraulic-fracturing treatment is a fluid conduit and has conductivity. This conductivity is responsible for the difference in the pre- and post-fracturing well productivity. In practice, the geometry of a hydraulically generated fracture is not known exactly; therefore, the actual fracture conductivity cannot be calculated directly. Pressure transient analysis of post-frac pressure buildup data can be used to estimate fracture dimensions and conductivity (Cikes 2000). Cikes used this method to demonstrate that fracture conductivity decreased dramatically in high-temperature wells that were propped with high-strength proppants. He suggested that there must be some unknown damage mechanism causing the dramatic conductivity decline even in the wells that were not on production.
Proppant Pack. The hydraulic conductivity of a proppant pack more than a monolayer thick is limited by the porosity of the pack. This is typically 38 to 42% for a well-classified proppant. Small changes in pack porosity result in significant changes in pack permeability and fracture conductivity. Fracture conductivity is designed by controlling concentration of proppant used to hold the fracture width open and is limited by the porosity of the pack. Certain surface modification agents (SMA) can be applied during frac treatments to enhance proppant pack porosity by causing the proppant to resist forming tight packs, resulting in higher than expected porosity.