Bubbling under at Bunga Raya
  from: Offshore Engineer
  Friday, September 05, 2008

A lightweight completion fluid was used by Talisman Malaysia to achieve the necessary underbalance to obtain clean perforation in a new offshore well. Results showed a measurable improvement over that of an adjacent well that had been perforated with a conventional underbalance fluid.

This work took place in April 2007 in Block PM3 of the Bunga Raya field, between Malaysia and Vietnam. According to Graeme Rae, head of well technology for Talisman Malaysia, reservoir pressures are relatively depleted in this field, a condition that increases the likelihood of wellbore damage in perforated completions where the invasion of pulverized rock formation grains can create an undesirable low permeability crushed zone and restrict production. He notes that underbalance is widely considered to be the most efficient means to achieve clean perforation under such conditions.

Lower permeability rocks, explains Rae, tend to exhibit a greater percentage of permeability reduction under conditions of overbalance. Perforating shock waves and high-impact pressure can shatter rock grains, break down inter-granular mineral cementation and de-bond clay particles to form a low-permeability crushed zone in the formation around perforation tunnels.

In contrast, an underbalance condition facilitates flow from the formation into the tunnel, and aids in the removal of perforating debris while at the same time minimising or eliminating crushed-zone damage.

Water injection resources are reportedly limited in this region, making it increasingly difficult to perforate new wells with the desired underbalance. According to Talisman Malaysia, traditional completion fluids have limited application in depleted reservoirs as the lowest available density is on the order of 6.6ppg, which represents an overbalance under local conditions.

Rae and his team began to consider the possibility of developing an alternate perforation fluid during a technical presentation on lightweight drilling fluids offered by an industry supplier. Based on the properties of the drilling fluid, he speculated that it might be possible to formulate a lightweight completion fluid with lower density to complement underbalance perforation. The supplier agreed to participate in a development effort with this objective.

‘We screened a number of lightweight fluid formulations in the process,’ says Rae. ‘Our objectives included achieving the lowest possible density along with good fluid stability, and an acceptable viscosity.’

Perforation fluid composition

Three different formulations of synthetic oil and glass bubbles with a rheology agent were tested for density, stability and viscosity values. The team eventually selected a composition consisting of 65% Shell Sarapar 147 synthetic oil, 35% 3M glass bubbles as a density reducing agent, and a rheology control agent amounting to 4% of the volume. This agent effectively suspends the glass bubbles to form a homogenous slurry, and the resulting fluid has an effective density of 5.5ppg.

‘The relatively low surface area and good strength-to-weight ratio of the glass bubbles provides the ability to achieve high “solids” loading in the base fluid system and thereby achieve the desired underbalance value,’ explains 3M Malaysia’s Melvin Devadass. ‘These soda-lime borosilicate glass bubbles are chemically inert, other than in the presence of HF, and exhibit the pressure and temperature resistance values required for this application.’

Studies showed that fluid stability is inversely proportional to the amount of glass bubbles in the mixture. For example, with only 1% glass bubble content the fluid exhibits very short stability, separating after only ten minutes. However, with increasing glass bubble loading the time to separation increases accordingly, and at the level of 28% content stability improves substantially. The rheology agent appears to form a network that agglomerates the bulk of glass bubbles in the mixture.

3M’s glass bubbles have established application in oil exploration and production for subsea riser flotation, syntactic foam insulation for subsea flowlines, in lightweight drilling fluids and as a component in reduced-density downhole cement, says Devadass.

Field tests

According to Talisman Malaysia, several operational issues were encountered during lightweight perforation fluid field tests.

In the first perforation test the fluid had not been properly sheared, and consequently it separated into distinct layers during preparation and could not be used. This separation was eventually traced to improper handling of the fluid rather than a deficiency in the formulation. In a second attempt, the fluid was pumped into the well, where it appeared to function as intended until the crew encountered high pickup weights. ‘At first it was thought that the perforation fluid had separated downhole, but upon circulating it out, the fluid was found to be in good condition,’ says Rae. ‘It was eventually determined that the high pickup weights were attributable to well deviation.’

Based on that experience, Talisman Malaysia engineers decided that in future, a pill of normal base oil should be placed first to assure a clean area before completion fluid is pumped downhole.

On the third completion attempt with lightweight underbalance fluid, which was at BKC-18, Rae reports that well perforation went smoothly and without operational difficulties. About 72 barrels of lightweight completion fluid was pumped into the well.

BKC-18 has a measured depth of 3005m, and a total vertical depth of 2058.3m, which classifies it as moderate to high-angle. This well is a single zone oil producer with a reservoir pressure of approximately 2200psi. Its orientation did not allow for wireline perforation where swabbing could be an option, and this made tubingconveyed perforation the most practical approach.

‘With the use of the low-weight completion fluid, we achieved an underbalanced condition of approximately 177psi,’ reports Talisman Malaysia.

‘The fluid pump rate was set between 0.5 and 1.0 barrels per minute, and completion proceeded without any issues and successfully perforated with the lightweight completion fluid in place.’

Production history of BKC-18 shows a marked increase in production rate compared to neighboring wells that produce from the same reservoir but were perforated by traditional means. Production was maintained at above 2000b/d after four months, while the output of adjacent BKC 17 dropped to below 1000b/d during the same period (see chart).

‘The laboratory development of a super low density completion fluid and subsequent perforation testing at BKC-18 was successful,’ notes Rae. ‘The only variable that can be assigned to the complete success of BKC-18 and its performance compared to associated wells is the use of the low-density completion fluid.

‘While this new lightweight perforation fluid is not a panacea for well completion in general, it adds to the arsenal of problem-solving options, and should be an effective tool for tunnel cleanup as similar low pressure reservoir circumstances are encountered in the future, whether in block PM3 or elsewhere,’ he concludes. OE

Click here to register to receive your own copy of Offshore Engineer each month.