Barite scale in hydraulic fracturing systems is a significant issue for overall oil/ gas production. Though it has long been assumed that the Ba necessary for scale formation is derived from the host rock, another potential and possibly dominant source often overlooked is Ba-rich drilling mud often used in these systems. Because drilling mud contains barite concentrations that are two orders of magnitude greater than Ba-rich oil/ gas shales (20-60 wt.% in drilling mud versus ~1,000 mg/ kg in Ba-rich shales), it is important to determine the potential of drilling mud as a source of both barium and sulfate in hydraulic fracturing systems. In order to assess the validity of drilling mud being a major/ primary Ba source, the stability of barite in drilling mud needs to be tested. A series of leaching experiments on pure barite and Ba-rich drilling mud were conducted under a variety of pH's and ionic strengths as well as in the presence of a wide variety of organics common in fracture fluid systems (additives, formation water, produced water) and compared to Ba release from four different oil/ gas shale plays.
In experiments where Ba was leached from various oil/ gas shales, the total amount of Ba released (< 50﹪ of total Ba) resulted in aqueous Ba concentrations far below levels commonly measured in produced water. Conversely, the leaching experiment using pure barite and more importantly the drilling mud resulted in significant releases of Ba under system-relevant conditions (low initial pH). In these experimental systems the resulting Ba concentrations were closer to those in produced waters. Drilling mud reacted with 15﹪ HCl (similar to that used in the initial acid slug) released > 18﹪ of the total Ba in the drilling mud (20-60 wt.%) over a 72-hour interval. It was found that under most chemical conditions the barite contained in the drilling mud is less stable compared to pure barite, suggesting that drilling mud used in unconventionals is a primary source of Ba in these systems. Because barite is less stable at low pH and high ionic strength, as injected acid is neutralized by the shale (during which both pH and ionic strength increase), a window of time where barite precipitation will occur due to the Ba and SO4 released from the drilling mud and transported into the newly formed fractures prior to cessation of precipitation due to I.S. increasing to over 1 M. This new scale precipitation will potentially close portions of the newly formed secondary porosity. Based on this work, we propose a new conceptual model for Ba cycling in hydraulic fracturing systems in which drilling mud is an important Ba source resulting in self-inflicted formation damage.
