An Experimental Study in Perforating and Gravel Packing Unconsolidated Sand Formations
Authors: Wong G.K. , Willson S.M. , Black A.D. 
Published: December 2007

This paper summarizes results of an experimental program developed and implemented to investigate the impact of perforating and gravel packing cavities in unconsolidated sands. The program contains a total of 16 single perforation tests, with each sample (15.5-in. diameter and 35-in. length) made from a representative sand, stressed to downhole stress conditions, shot with actual field size charges (22.7 gm big hole), gravel packed, and then produced to measure the productivity of the completion. our paper's objective is to evaluate perforating conditions that result in more effective perforation. prepack or gravel-pack operations in unconsolidated sand reservoirs as a single system. Results from our tests showed no perforation cavity in unconsolidated sand. Pressure recorded at the wellbore suggested that sand fluidization may be taking place during perforation causing the cavity to collapse. Packing the collapsed perforation cavity on a consist basis was difficult to achieve as the condition of formation/cement interface seems to play an important role. Overbalance perforating without backsurging resulted in productivity seven times less than the case with backsurging. This paper summarizes key observations and findings of the testing program.

Introduction

Unconsolidated sand reservoirs normally require sand control. For cased hole wells, the most common sand control completion is the internal gravel pack (IGP). This generally involves underbalance tubing conveyed perforating (TCP) followed by gravel packing (GP). Improving the inflow performance of IGP (while maintaining sand control) is essential for maximizing the well productivity. This depends on (1) having sufficient perforation flow area (high shot density and large perforation diameter), (2) effectively removing or cleaning the perforation debris and crushed grains (fines) from the perforating jetting action (proper underbalance pressure and flow volume), and (3) packing the perforation cavities in the formation, the tunnels in the casing/cement sheath, and the casing/screen annulus volume with high-permeability gravels.

The problem of cleaning and then packing the perforation (cavity in the formation and tunnel in the casing/cement sheath) needs to be evaluated as a single system. A higher underbalance pressure may remove more fines and perforation debris. In unconsolidated sands a higher underbalance can also increase the potential of collapsing perforation cavities which become more difficult to gravel pack. On the other hand, if one uses a lower underbalance pressure to produce cavities that are open and easier to gravel pack, the productivity of such a completion may be compromised because of ineffective perforation cleanup.

Available physical modeling studies of GP completion generally use cylindrical cavities to emulate the perforation cavities in the formation. This assumption of open perforation cavities for unconsolidated sands may not be valid and warrants review. Blok, et al. used single shot perforating and gravel packing experiments to evaluate the productivity of IGP for a weakly consolidated rock (Castlegate sandstone with unconfined strength of 1200 psi). They were able to observe open perforation cavities and highlighted the importance of perforation cleanup in maintaining the productivity of IGP completion. Similar laboratory data for unconsolidated sands are not available in the literature and the existence of perforation cavities in unconsolidated sand formations has not been demonstrated.

This paper presents results of an experimental program developed and implemented to investigate the impact of perforating and gravel packing cavities in unconsolidated sands. P. 649^