Perforating In Tight Sandstones: Effect Of Pore Fluid And Underbalance
Authors: Bird K. , Blok R.H.J. 
Published: December 2007

Abstract
Single shot perforation and flow experiments have been carried out in gas saturated and brine saturated low permeability sandstone (kbrine&lt; 1 mD), to investigate the effect of pore fluid type (liquid/gas) and underbalance on perforation productivity and geometry. A total of 16 cores (8 nitrogen saturated and 8 brine saturated) were perforated with deep penetrating charges under simulated downhole conditions at pressures ranging from 100 psi. overbalance to 2,000 psi. underbalance. Only one of the liquid saturated cores eventually cleaned up fully. Only for gas saturated cores was a clear relationship between the level of overbalance/underbalance and productivity observed.
The experiments demonstrate that high underbalance conditions are required to maximise inflow performance when perforating in tight sandstones, particularly in liquid saturated formations. The results also have implications for hydraulic fracturing operations where often the effectiveness of perforating is considered of secondary importance. Insufficient underbalance and poor perforation clean up may lead to unexpectedly high formation breakdown pressures and related operational difficulties.
This paper includes a detailed examination of the perforated cores, including the results of x-ray scans, flow performance analysis and microscopic examination of the perforation tunnel and perforation damage zone.

Introduction
Evaluation of the performance of perforating charges under downhole conditions has advanced over the past few years as a result of the arrival of perforating test equipment that meets the design criteria specified in API Recommended Practice 43. Cores of outcrop material, stressed to equivalent overburden stress, are perforated and flow tested to determine the influence of various parameters on perforation geometry and flow performance. A large data set of perforation performance in liquid saturated targets is nowadays available. Development of low permeability gas reservoirs requires more insight into perforator performance in gas saturated rock. Following the commissioning of a new perforating facility at the British Gas Research Station in Newcastle Upon Tyne, U.K., a programme was set up to test perforator performance in gas saturated, tight sandstone. To allow direct comparison with perforator performance in liquid saturated rock, 16 cores (8 nitrogen saturated and 8 brine saturated) were perforated under various levels of underbalance ranging from 100 psi. overbalance to 2,000 psi. underbalance. The objectives of the programme were to establish the optimum level of underbalance while perforating for the field type under study and to investigate the impact of pore fluid type (liquid/gas) on the perforation damage process.
The results of the tests showed that full perforation clean up in this type of rock is difficult to achieve both in gas and liquid saturated cores, even at high underbalance levels (2,000 psi.). In gas saturated cores the level of underbalance could be correlated with the degree of clean up, therefore the maximum level of underbalance during perforating that can be achieved was recommended to give the best inflow performance. Total penetration was greater in the brine saturated cores than in the gas saturated cores, however, the gas saturated cores cleaned up better which resulted in deeper debris-free tunnels. In brine saturated cores no clear correlation was observed between the level of underbalance and the flow performance.

Testing
Test Programme. The test programme is given in Table 1. It was structured to provide as much information on the effect of underbalance and fluid type (liquid/gas) on perforation productivity and geometry, therefore these were the only parameters varied during testing. The same simulated downhole rock stresses and charge type were used throughout. P. 385