Abstract

Proppant conductivity is an important design criterion in hydraulic fracturing treatments. Knowing how different proppants behave under changing stress conditions is important to fracture stimulation success. Conductivity and non-Darcy flow effects has been laboratory measured for all ceramic proppants. Unfortunately, almost all laboratory measurements are performed with an increasing stress and cyclic stress behavior is not observed. Often, in the production of oil and gas wells, shut-in periods occur and pressure in the wellbore and proppant pack increases causing stress to be relieved on the pack. When production begins again, stress is increased on the pack. This is stress cycling and past publications¹ have noted that stress cycling can cause a reduction in proppant pack conductivity.

The work presented in this paper chronicles a vast series of crush tests performed on several proppant types and sizes. The crush tests were run using exactly the same starting sieve distribution for each particular proppant tested. After a sample was subjected to stress or multiple stress cycles, it was sieved for a detailed analysis. The change in the sieve distribution was noted and a median particle diameter was calculated along with the standard deviation of the distribution. A relationship was observed between these values and those of conductivity and Beta Factor. From these relationships it can then be possible to estimate the effect that stresses cycling has on conductivity and non-Darcy flow effects.

The papers results can help the fracture design engineer to better understand how conductivity, of the designed proppant pack, will change with time as stress cycling occurs. This understanding can lead to the recovery of more oil and gas through improved stimulation results.

Introduction

Conductivity is a very important consideration in the design of hydraulic fracture treatments. Many publications have documented this to one degree or another^2,3,4. The conductivity that is achieved from a placed hydraulic fracture treatment is a result of many factors. Fracture closure stress, proppant pack concentration, proppant type, proppant sieve distribution, proppant embedment, gel residue damage, stress cycling and others all affect the proppant pack conductivity that can be achieved from a treatment.

Baseline conductivity properties, evaluated using API standards⁵, are available for most proppants available for use in hydraulic fracture treatments. These conductivity tests are performed under controlled laboratory conditions and usually represent the maximum attainable conductivity for any particular proppant for the conditions tested.

Proppants come in many different sizes and materials. Some are naturally occurring, while others are man made. Some have coatings to increase cohesiveness and strength. Most proppant sizes are referred to by an API sieve size designation. For instance, a 20/40 mesh proppant would have a sieve distribution in which 90% would fall through the 20 mesh sieve and stay on the 40 mesh sieve. Other typical API proppant designations are 30/50 mesh, 16/30 mesh, 12/18 mesh, 16/20 mesh, and on and on. Unfortunately, all 20/40 mesh proppants do not have the same sieve distribution and can vary greatly in Median Particle Diameter (MPD) depending on whether the sieve distribution is weighted more on the coarse or fine end of the 20 to 40 mesh scale.

The importance of MPD becomes apparent in Fig. 1 which shows a plot of MPD versus baseline conductivity at 2,000 psi closure for a wide variety of commercially available ceramic proppants. From this graph we see that conductivity is very strongly related to MPD. As stress increases on the proppant pack, individual proppant balls begin to crush and become smaller pieces. This is why the baseline conductivity at 2,000 psi was chosen, very little proppant crushing, if any, should be occurring at this stress with ceramic proppants. The graph also seems to indicate that the type ceramic material is not as important to conductivity as MPD, since all proppant materials plot on the same trend.