RIASSUNTO
Abstract
In a number of mature gas fields in the Southern North Sea there are several multilateral open-hole completions that have proven to give significant solids/pebble production problems. Since the onset of production six out of seven total multilateral open-hole gas wells had to be shut in because of solids plugging. Collapse of the open hole multi-lateral junction is assumed to be the root cause of these problems. Wells that were drilled later in field life equipped with a pre-drilled liner did not show this solids production problem. There was, however, no way (short of a full workover to recomplete the well) to retrofit a solids control measure of this type until now. A coiled tubing cleanout campaign was commissioned to restore productivity of these plugged gas wells. After the cleanouts perforated jointed pipes have been installed in the bottom of the existing completion as a solids production prevention mechanism. This was done under live well conditions, without having to resort to snubbing. A newly developed special pipe connector was used to build in this perforated pipe section using a combination of braided wire and coiled tubing. Cleanout and restoration of gas production was successful for four out of the total six plugged wells. In conclusion, new connector technology has been instrumental in retrofitting a means of subsurface solids control in live gas wells with openhole original completions.
Background: Tight gas wells, with problems
In the UK part of the Southern North Sea average permeabilities of less than 1 mD are considered ‘tight’ (Ref. 1). Improving gas well capacities has been the number one challenge in developing several tight gas fields in this region. Over the years different approaches were taken to improve well capacities and maximize field ultimate recovery. Wells drilled in the 1980s were predominantly hydraulically fractured (Ref. 2). Challenges experienced were related to production of proppant, fracturing into the water leg causing early water breakthrough, and poor cleanup.
Starting in the 1990s horizontal wells became very successful and overtook the vertical hydraulically fractured wells in productivity by a factor between two and eight. The main driver for the success of horizontal wells was increasing the odds of intersecting of natural fracture systems. Of course, losses while drilling through the open natural fractures would often limit the wellbore length and the number of natural fractures intersected.
Underbalanced drilling (UBD) was introduced in the mid 1990s to prevent losses and damage to natural fractures. An important added benefit of underbalanced drilling was the monitoring of well productivity while drilling (see Figure 1). The PIWD (Productivity Index While Drilling) (illustrated in Figure 2) shows sharp increases as productive features are being drilled (see also Ref. 3). In some cases multiple sidetracks were required before a high enough PIWD was reached. Of course the UBD technique did not come without challenges. The most important one was the fact that at the time the first UBD wells were drilled there was no means of installing solids control in the form of a screen or pre-drilled liner in ‘underbalanced mode’. UBD wells were completed barefoot and a gradual beanup policy was implemented to try and prevent massive solids production. In spite of the slow beanup many of the UBD wells developed problems with solids production, sometime producing large pebbles to surface or plugging up parts of the completion (see Figure 3). Suspected cause of the solids production was either collapse of the openhole, or collapse of the multilateral junctions (see Figure 4). The shape and size of the pebbles made it less likely that they had broken off deeper in the well. The cause of junction collapse could be the concentration of stress at the start of the junction. The hole collapses occurred mostly early in well life. Table 1 shows a summary of the plugged completions, all of them drilled underbalanced, barefoot wells. Attempts to bail solids were carried out using wireline bailing removing bridges from the tubing. One case was successful, but the well sanded up again 3 years later after an emergency shutdown.