Effect of the Wellbore Conditions on the Performance of Underground Gas-Storage Reservoirs

Abstract

Abstract This paper presents results of an underground gas storage modeling study using the reservoir and fluid properties of the Northern Marmara gas field, a recently developed field in the Thrace region of Turkey. It has an area of 3.6 km2 with nearly 3 billion sm3 of GIIP. The reservoir model used in this study is based on the theoretical as well as the empirical deliverability equations. The deliverability equations reflect the production and injection performances of the wells with given completion characteristics. Our approach is based on maximizing working gas volume for a particular configuration of reservoir, well and surface facilities. The importance of the type of well completion was emphasized in this paper because this is the most likely place for operating problems to occur. The deliverability performance strongly affects the design factors such as the working gas volume and the number of wells operated. Results indicate the performance dependence on wellbore damage and hence the importance of cleanup of wells. The effects of the mechanical skin factor on the working gas capacity with different number of horizontal wells as well as vertical ones were studied. Results show that the working gas capacity decreases while the mechanical skin factor increases. This behavior becomes especially more dramatic as the horizontal well length decreases. The increasing number of wells increases the working gas capacity as expected. These results are discussed in detail through application to N. Marmara gas reservoir data. The underground gas storage modelling approach followed in this study is shown to be an effective way to forecast the production / injection performance and to estimate the important design parameters when converting a depleted gas reservoir to a gas storage reservoir. Our approach yields the better understanding of the effect of the wellbore conditions on the performance of the underground gas storage reservoirs as well. Introduction Underground Storage of Natural Gas. With the rapid growth of the natural gas industry, underground storage of natural gas has grown to become a large and essential part of the natural gas delivery system. Long-term demand variations of natural gas caused by the increased fuel need for space heating during cold weather, require large amounts to be stored. These seasonal demand variations can be satisfied effectively by underground natural gas storage, if such facilities exist close to the area where demand variations take place. Underground storage of natural gas is a very well known practice to gas utilities, gas producers and large ultimate gas consumers for mainly economical reasons. What is done in underground storage is simply the injection of the excess supply obtained during the summer season into a depleted oil or gas field and producing it during peak load periods.1 Purpose of Underground Natural Gas Storage. Storage over the past years has served a number of purposes and the developing industry is still finding new roles for it. The primary objective of underground storage is to provide an economical way to supply gas for space heating consumption. Furthermore storage allows the transmission lines to be used at full capacity during the entire year by delivering gas to the consumer or to the underground storage reservoir. Also the storage field can be used to store gas from low-pressured wells enabling storage wells to have a much greater deliverability during the peak season. In a case where a great portion of the country's supply is maintained from long distance transmission lines, underground storage acts as a safeguard for possible pipeline failures thus becomes a strategic reserve. A final major advantage of underground storage is safety. Placement of gas underground in a protected oxygen-free environment greatly reduces the risks of a fire or explosion. Underground Storage of Natural Gas. With the rapid growth of the natural gas industry, underground storage of natural gas has grown to become a large and essential part of the natural gas delivery system. Long-term demand variations of natural gas caused by the increased fuel need for space heating during cold weather, require large amounts to be stored. These seasonal demand variations can be satisfied effectively by underground natural gas storage, if such facilities exist close to the area where demand variations take place. Underground storage of natural gas is a very well known practice to gas utilities, gas producers and large ultimate gas consumers for mainly economical reasons. What is done in underground storage is simply the injection of the excess supply obtained during the summer season into a depleted oil or gas field and producing it during peak load periods.1 Purpose of Underground Natural Gas Storage. Storage over the past years has served a number of purposes and the developing industry is still finding new roles for it. The primary objective of underground storage is to provide an economical way to supply gas for space heating consumption. Furthermore storage allows the transmission lines to be used at full capacity during the entire year by delivering gas to the consumer or to the underground storage reservoir. Also the storage field can be used to store gas from low-pressured wells enabling storage wells to have a much greater deliverability during the peak season. In a case where a great portion of the country's supply is maintained from long distance transmission lines, underground storage acts as a safeguard for possible pipeline failures thus becomes a strategic reserve. A final major advantage of underground storage is safety. Placement of gas underground in a protected oxygen-free environment greatly reduces the risks of a fire or explosion.