Archives of Wellbore Impression Data From Openhole Vertical Well Fracs in the Late 1960's

摘要

During the timeframe of the mid- to late 1960's, a large number of those working in the Oil and Gas industry were still of the mindset that most hydraulic fracturing treatments were creating horizontal fractures, although the bulk of the technical papers still available from this time period seem to mostly assume or even verify vertical fractures. This view seemed especially prevalent for the numerous sandstone reservoirs that contained thin laminations of shale or shaley sand that seemed to have only very weak bonding to the adjacent sand layers. Many of this group had been present when such a formation was being cored, and observed the core sample simply separate at a (horizontal) bedding plane interface or a thin shale streak when the core would be removed from the core barrel. Many others had read more than one core analysis report containing similar descriptions of this occurring to cores when they were being tested. This misguided "horizontal fracturing concept" was almost always in disagreement with observed pressure data during fracturing operations. Although pressure gradients during breakdown of cased and perforated wells occasionally would approach the reservoir's overburden pressure, fracture extension pressures almost never would. Starting in about the mid-1960's, in an effort to enhance acceptance among the "rank and file" group in oilfield operations that vertical fracture orientation was both expected and actual outcome for almost every well, or at least when deeper than the 1,500 to 2,000 ft range at least one service operator and numerous well operators joined efforts to make rubber impression molds of openhole wellbores immediately after the well had been proppant-fracture stimulated. This was soon followed by adding a method to identify the compass orientation and investigate a more technical purpose: to document the fracture direction. Unfortunately, only a small part of the resulting pictorial data base was ever published into the preserved public domain during the decade of their significant use. This paper will revisit the few known papers within the literature and also data archives preserved in internal company reports and by nearly a hundred photographs of created rubber wellbore molds following openhole hydraulic fracture treatments. These show the resultant propped fracture traces inside wellbores from more than a dozen openhole applications of this simple technology. Such results as fractures (at 4° to 8° angles off of vertical) may only be present inside the wellbore for a few feet, and numerous cases of clear evidence of two and three separate parallel propped fracs present at least at the wellbore wall.