"Super Height Growth"—Strange Surface Pressures and Refrac Potential
Authors: Gil Ivan R. , Smith Michael B. , Haddad Ziad , Lawrence Perry , Lestz Robert , Schott David , Reese Jennifer 
Published: December 2007

"Strange", increasing, surface pressure data is commonly observed after shut-in of a fracturing injection in some areas. What causes this? Another common question today is "What is the potential for re-fracs in this field?" This paper discusses how both questions relate to fracture geometry termed ‘Super Height Growth’ – rate of height growth exceeding rate of length growth.

Often, when we find situations with minimal height confinement, we refer to "radial fractures", i.e., frac tip-to-tip length = total height. Indeed, this type geometry, in some environments, is commonly predicted by Pseudo-3D models. However, one consequence of this assumed shape is that more pumping continues to create length. If, instead of radial, conditions caused vertical propagation to dominate, then an assumption that continued pumping increases length is invalid - with huge impact on designs.

Previous studies (Lestz, et. al., 2002) have shown that fractures in South Texas are often dominated by height growth – i.e., "radial" fractures. However, data collection over several years in the South Texas Wilcox & Vicksburg indicates that in many areas, geometry is actually dominated by "Super Height Growth". After some critical point, geometry is completely dominated by vertical growth – length growth ceases. Designs have systematically changed to smaller treatments (with much smaller pad volumes) to reflect this "understanding" – with general success in that no apparent decrease in effects has been noted despite job volume (particularly proppant & pad) reductions on the order of 70%. Note that data collected has consisted of in situ stress measured in multiple wells (including shale stress tests), pre-frac injection testing, net treating pressure data from fracture treatments, tracer logs, and one post-frac pressure build-up test (showing fracture length even less than expected). It appears that "super" height growth behavior places an absolute limit on the propped fracture length than can be created. In such cases, a small degree of reservoir depletion (and the resulting decrease in formation stress) may provide an excellent re-frac candidate!

A common phenomena in this area are "strange", increasing surface pressures after shut-in of a mini-frac and/or frac. This has also been discussed in the literature, and this behavior is often "blamed" on "thermal effects". However, thermal effects cannot cause pressure increases if there is good communication to an open fracture – implying that the fracture closed in minutes (or seconds) after the end of pumping. That seems unlikely in these low loss formations. Thus, "What causes the pressure increase?" The paper reviews such data and explores how the concept of "Super Height Growth" can explain this post-frac behavior, and discussed the implications of this on fracture analysis/design.

A combination of these two things: "strange" increasing pressure indicating "Super Height Growth", and the effect of this geometry on limiting fracture penetration may then provide excellent re-frac candidates. This is discussed and several brief case histories are presented.