One major source of new natural gas supplies for the world will be gas from low permeability reservoirs. Currently, natural gas from tight sandstones, carbonates, coal seams and shales account for over 25% of the annual gas production in the United States. Global production of gas from similar reservoirs world wide will be expected in the next few decades. For such reservoirs, the wells have to be successfully fracture treated to produce at commercial gas flow rates.
In many situations, the created length was probably achieved, and may be even the propped length was achieved. However, due to insufficient proppant concentration, or insufficient proppant transport, or the use of the wrong propping agent, or a fracture fluid that does not break to a low viscosity fluid, the effective fracture length is significantly less than the designed length and does not provide optimal production results.
It is our opinion that insufficient fracture fluid cleanup is the primary cause of poor results when the optimal effective fracture length is not achieved. Fracture fluid cleanup is affected by gel damage, which includes the static yield stress, the flowing yield stress, the amount of polymer residue in the fracture after the fracture closes, and the amount of polymer that forms a filter cake on the walls of the fracture. In addition, there are other issues such as proppant crushing and non-Darcy flow effects that also affect fracture fluid cleanup.
In this research, we have used a reservoir simulator to model how polymers in the fracture affect fracture fluid cleanup. We have incorporated mathematical expressions for static yield stress. We have included the effects of proppant crushing, gel residue plugging and the formation of a filter cake on the clean up behavior and the resulting gas flow rates. With our model, we are able to simulate many of the problems we observe in field data and problems documented in the petroleum literature. Even though real solutions to these problems still remain to be developed, we think we can explain the issues behind most fracture fluid cleanup problems and offer a few ideas on what can be done to solve them.
A tight gas reservoir is a low-porosity, low-permeability formation that must be fracture treated to flow at economic gas rates and to recover economic volumes of gas. Without a fracture treatment, gas from tight gas reservoirs will produce at low flow rates under radial flow conditions. After a successful fracture treatment, the gas flow mechanism in the reservoir will change from radial flow to linear flow as shown in Fig. 1. As the propped fracture length increases, the well will produce more gas at higher flow rates provided that adequate fracture conductivity is also created.