Investigating the Impact of Dynamic Effective Stress While Perforating Low-Strength Formations
Authors: Haggerty Dennis , Craddock Gerald , McGregor Jacob 
Published: January 2018

Dynamic effective stress (DES) is a consequence of dynamic underbalance (UB) (Bell 1984) created when opening a lower-pressured wellbore by means of perforation to a higher-pressure formation to promote perforation cleanup, if intentional. It pertains to the brief, rapid rise in formation effective stress around newly created perforation tunnel walls immediately following the detonation of tubing-conveyed perforating (TCP) guns. DES is the by-product of the dynamic UB commonly enhanced to surge debris out of the perforations with the rapid movement of formation fluid toward the wellbore (Bolchover and Walton 2006). In step with the dynamic UB, the DES value is the point at which the wellbore reaches its lowest pressure, and the DES event typically occurs for less than one second during the surge of pressurized formation fluid toward the 1-atm hollow-carrier perforating gun inside the cased wellbore. By understanding this mechanism, successful gravel packing may be achievable in a low-strength formation, displacing loose perforating debris to create space for high permeability gravel. Important parameters that define the degree of dynamic UB to expect are the same for DES and include formation pressure, shaped charge and gun characteristics, and formation rock and fluid properties. During advanced American Petroleum Institute Recommended Practice 19B Section 2/4 laboratory (API RP 19B 2014) for perforator performance testing, these parameters can be matched at pressures and temperatures where dynamic UB and DES effects are intensified. By conducting multiple tests adjusting the laboratory gun void space, simulated-wellbore volume, type and size of the shaped charge, and formation target from low-strength to moderate-strength, observations can be analyzed and quantified.

It has been observed that when perforating in weak rock with less than 1,000 psi unconfined compressive strength (UCS), even the smallest DES increase with associated dynamic UB flow can result in a collapsed tunnel or filled-in cavity, as viewed in computed tomography (CT) scan images. Conversely, perforation tunnels in rocks with higher UCS have been observed to withstand collapse when subjected to a high DES and dynamic UB. The variance of rock strength and differences in shaped-charge design determine the maximum DES and dynamic UB that are tolerable in a specific wellbore-matched configuration. Another observation is that the viability of the perforation tunnel depends heavily on the size of the exit hole diameter created in the casing. Large, big hole (BH) charges creating upward of 1-in. diameter holes in the casing require larger-diameter rock targets for examining DES effects in low strength, high-flow-capacity reservoir-analog rocks. This is to ensure survival of the shock at detonation and avoidance of boundary failure. Typical 7-in. diameter rock targets of 2 to 3 ft in length have proven to be adequate for examining DES effects on the current largest deep penetrating (DP) charge, generally designed for higher UCS (higher strength) rocks. In addition, oil vs. water pore space fluid shows slightly more resistance to perforation tunnel collapse. The identification of the conditions for creating and preserving a debris-free tunnel during the perforating process is important for the success of subsequent steps in the completion process, such as gravel packing or stimulating. Performing customized API RP 19B Section 2/4 testing at the unique conditions of the specific well under scrutiny can help to guide completion engineers to select the best perforating technique.