A method to calculate the annular liquid volume of an immobile string

Immobile string technology is often used to develop the production of multi-layer and thin low-permeability gas reservoirs. This type of reservoir is characterized by low productivities and high pressure drop rates. Simplified ground facilities and control choke technologies are often adopted to effectively reduce the investment. Because gas wells in low permeability gas reservoirs often have low productivities (for instance, the individual well production rates in the Sulige Gas Field range from only 3000 m3/d to 5000 m3/d), a large number of adopted down-hole sensors will result in gas fields that are unable to run properly. The water production data and the bottom pressure become difficult to measure. The casing pressure is controlled by both reservoir pressure and liquid accumulation. To predict liquid volume correctly, the superposed effects of the reservoir pressure and liquid accumulation should be stripped as a priority. Taking the dynamic reserve, production proration and production rate into consideration, we analyzed the casing pressures of dry gas wells. In addition, we quoted a parameter (the production rate in the unit casing pressure drop) to remove the influence of reservoir pressure drop and the output fluctuation “noise” on casing pressure. To strip out the effects of liquid accumulation, we discussed the casing pressure under different liquid accumulation rates. Through the above research, we created a method to calculate the annular liquid volume of an immobile string. This method applied to the Sulige Gas Field very well, and it can also be introduced to other similar gas reservoirs.