Experimental Investigation of Chalk Influx in a Depleted Reservoir

Chalk is generally a weak reservoir rock with high porosity and moderately low permeability compared to sandstone reservoir rock. Chalk production and chalk influx events are a challenge that operators encounter during the production of hydrocarbons from chalk reservoirs. Appropriate completion of the well is therefore crucial for long term optimal production. The challenges are bigger when producing from a depleted reservoir. To avoid problems during production and increase the lifetime of the producing reservoir, it is crucial to comprehend the behavior of chalk production and/or chalk influx in a depleted reservoir. A series of experiments has been undertaken to investigate this behavior in simulated depleted reservoirs using outcrop Mons chalk as testing material in laboratory experiments. The tests were conducted in two phases. The first phase involved simulation of depletion at two different stress levels followed by re-pressurization and the second phase replicated production by increasing the applied drawdown. The results indicate a reduction in drawdown stress within a depleted reservoir compared to a non-depleted one. It was also found that the critical drawdown is independent of the effective stress amplitude. 1. INTRODUCTION Weak chalk has a strong tendency to compact when the hydrocarbon is produced due to an increase in effective stresses. The compaction observed in the high porosity chalk reservoir can be extended to the sea floor surface; one well-known example of this phenomenon is the surface subsidence observed in the Ekofisk Field in the North Sea. Oil companies employ several reservoir management systems to prevent such situations. One strategy involves increasing the pore pressure in the reservoir to prevent reservoir compaction and enhance oil recovery. However, the high porosity and compressible nature of the reservoir pose significant challenges (Omdal, 2010). Drilling new wells to produce from such depleted and re-pressurized reservoirs is technically demanding in terms of borehole stability. Production of hydrocarbons from depleted and re-pressurized reservoirs adds another level of complication to an already challenging reservoir management system. Chalk influx or liquefaction in the form of toothpaste like material involves a significant part of chalk formation near the borehole which eventually fills the well, tubing and destroys the flow (Kristiansen and Meling 1996). It is important to understand the behavior and mechanisms of chalk influx to avoid or effectively manage chalk influx events to ensure uninterrupted production and maximize recovery.