Dynamic underbalanced perforating is a technique for obtaining clean perforation tunnels resulting in much improved production or water injection. This technique has been successfully applied to several wells in the Ninian Field in the UK sector of the North Sea, resulting not only in better wells, but also in increased completion efficiency. In August 2003 a closed-chamber, dynamic underbalance shoot and pull operation was performed on Ninian North—a world first. The well came in at 50% higher production than expected. Since then, several other wells have been perforated using the same technique. Gauge data on the early wells showed the sequence of events, but did not capture the detail. A more recent well has been perforated using a new high-speed gauge that revealed the extent of dynamic underbalanced achieved. This paper describes the method, shows the results and discusses the value of using this technique.
The Ninian field is located 130 km northeast of the Shetland Islands in the UK sector of the Northern North Sea. Ninian is a mature field discovered in 1974 with first oil in 1978. Production is from the Middle Jurasic Brent Group sandstones via one concrete and two steel platforms.
A typical completion has a cemented and perforated liner. Because of the length of the gross intervals that are perforated and depth of the wells, tubing conveyed perforating (TCP) systems are used.
With TCP techniques, the traditional method of perforating is to start with the well underbalanced to pore pressure, fire the guns and then flow the well to clean up before removing the guns. The amount of underbalance required to achieve perfectly clean perforations depends on several factors including rock strength, insitu stress, permeability, porosity, and fluid type. Most of these parameters are well known in mature assests. However, reservoir pressure is often not measured during open hole logging, and so estimates are used from reservoir models. Therefore, insitu stress is not known, placing an uncertainty on static underbalance requirements. More importantly, establishing the correct static underbalance before perforating is impossible if pore pressure is unknown.
Creating the optimum underbalance condition often involves changing or displacing completion fluids. This can be time consuming, expensive, and, in some cases, impossible to achieve. If this stage can be avoided, then the perforating process can become more efficient and save cost.
With dynamic underbalanced perforating, it is not necessary to start with the well underbalanced. In fact with TCP systems, the well can be overbalanced. So far, dynamic underbalanced perforating seems to give very good results across a wide range of permeabilities, porosities, pore pressures and well types—oil or gas producers, and water injectors[3, 4, 5]. In many cases results are much better than static underbalanced perforating and certainly no worse.
Static Underbalanced perforating has long been recognized as the best method for obtaining clean perforations. Since the introduction of TCP systems, static underbalanced perforating has become common practice. Many papers have been written on laboratory experiments to determine the optimum static underbalance and on field experience measuring the success of this technique[1, 6, 7, 8, 9]. The most widely used criteria within Schlumberger is the minimum underbalance equation derived from experimental work by Behrmann. This equation is included in the Schlumberger Perforating Analysis (SPAN) program and commonly used in the industry. The program is also used to calculate single-shot skin if the optimum underbalance is not used. The term single-shot skin is often expressed as a reduction in permeability around the perforation cavity (Kc). The ratio of this permeability to the virgin rock permeability (Kc/K) is used in both SPAN and nodal analysis programs.