Testing of Horizontal Gas Wells in, Anisotropic Naturally Fractured Reservoirs
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Horizontal wells are getting more and more widely used in oil and gas fields development due to plenty of its merits. However, there are examples when horizontal well drilling did not justify hopes. It is shown in this paper, that layered and anisotropic structure of the reservoir may lead to decrease of initial oil flow rates of horizontal wells and final oil recovery factors (ORF) reduction too. It is proved that pseudo-horizontal wells may incorporate advantages of vertical and horizontal wells. As a result, these wells may be more efficient in comparison with both horizontal and vertical wells. Introduction It is known that the first two horizontal well bores with 7 meter length in one well were drilled in Texas, 1930. But real breakthrough of horizontal and multilateral wells application was achieved in the USSR at the beginning of fifties. It was fulfilled under leadership of A.M. Grigorian. Simultaneously and even earlier, theoretical basis of oil field development by horizontal and multilateral wells was worked out. The interest to horizontal wells falls down to zero level in the USSR until sixties. From the beginning of eighties the idea of horizontal wells usage was regenerated in USA, France and other countries and nowadays it predominates all over the world including Russia. A lot of papers have recently been paid attention to flow theory conformably to horizontal wells. First of all, a number of papers were devoted to steady-state and transient flow of homogeneous fluid to horizontal well in homogeneously-anisotropic reservoir. These papers contain also corresponding methods of well test data interpretation. The theory of horizontal wells implementation in computer programs is given in papers. Detached collection of analogous papers is devoted to inclined wells. The problem of coning is investigated in the set of papers for the case of horizontal well usage. A lot of papers deal with theoretical aspects of horizontal wells application for different methods of influencing on fields, particularly, containing heavy, high-viscosity oils. There are papers considering and making proposals of development systems by means of horizontal wells. And at last, some papers set forth experience of horizontal well usage in real fields. At the same time there are a lot of cases of unsuccessful horizontal wells application and negative examples of horizontal wells usage are devoted very few attention. The authors would like to show, that layered heterogeneity is decisive factor, influencing on effectiveness of horizontal well application. This is obvious for everybody in the case of isolated layers. It is less evident in the case of crossflow between layers. After illustrating given factor's significance the paper makes attempt to prove pseudo-horizontal wells attractiveness. Reservoir model Numerical simulation is a tool of investigations presented. Element of layered formation is considered to be an object of corresponding studies. For simplicity we will consider a reservoir element, consisting of two layers with common thickness equal to 30 m. In the number of variants the upper layer has thickness equal to 10 m, in the rest examples - 20 m (see fig. 1). The most frequent values of permeabilities kh of the first and second layer are equal to 0,6 D and 0,2 D respectively. More precise information regarding properties of the layers, including anisotropy, is given in tabl. 1. On the front side of the considered reservoir vertical or horizontal producing well is located. Vertical well in all test cases completes both layers, horizontal well is variably placed in the upper or lower layer. The location of horizontal well is reflected in tabl. 1. The width of the reservoir is equal to 1200 m, the length of horizontal well - 600 m. On the back side of the reservoir vertical or horizontal injection well is symmetrically placed. P. 433
Horizontal wells are getting more and more widely used in oil and gas fields development due to plenty of its merits. However, there are examples when horizontal well drilling did not justify hopes. It is shown in this paper, that layered and anisotropic structure of the reservoir may lead to decrease of initial oil flow rates of horizontal wells and final oil recovery factors (ORF) reduction too. It is proved that pseudo-horizontal wells may incorporate advantages of vertical and horizontal wells. As a result, these wells may be more efficient in comparison with both horizontal and vertical wells. Introduction It is known that the first two horizontal well bores with 7 meter length in one well were drilled in Texas, 1930. But real breakthrough of horizontal and multilateral wells application was achieved in the USSR at the beginning of fifties. It was fulfilled under leadership of A.M. Grigorian. Simultaneously and even earlier, theoretical basis of oil field development by horizontal and multilateral wells was worked out. The interest to horizontal wells falls down to zero level in the USSR until sixties. From the beginning of eighties the idea of horizontal wells usage was regenerated in USA, France and other countries and nowadays it predominates all over the world including Russia. A lot of papers have recently been paid attention to flow theory conformably to horizontal wells. First of all, a number of papers were devoted to steady-state and transient flow of homogeneous fluid to horizontal well in homogeneously-anisotropic reservoir. These papers contain also corresponding methods of well test data interpretation. The theory of horizontal wells implementation in computer programs is given in papers. Detached collection of analogous papers is devoted to inclined wells. The problem of coning is investigated in the set of papers for the case of horizontal well usage. A lot of papers deal with theoretical aspects of horizontal wells application for different methods of influencing on fields, particularly, containing heavy, high-viscosity oils. There are papers considering and making proposals of development systems by means of horizontal wells. And at last, some papers set forth experience of horizontal well usage in real fields. At the same time there are a lot of cases of unsuccessful horizontal wells application and negative examples of horizontal wells usage are devoted very few attention. The authors would like to show, that layered heterogeneity is decisive factor, influencing on effectiveness of horizontal well application. This is obvious for everybody in the case of isolated layers. It is less evident in the case of crossflow between layers. After illustrating given factor's significance the paper makes attempt to prove pseudo-horizontal wells attractiveness. Reservoir model Numerical simulation is a tool of investigations presented. Element of layered formation is considered to be an object of corresponding studies. For simplicity we will consider a reservoir element, consisting of two layers with common thickness equal to 30 m. In the number of variants the upper layer has thickness equal to 10 m, in the rest examples - 20 m (see fig. 1). The most frequent values of permeabilities kh of the first and second layer are equal to 0,6 D and 0,2 D respectively. More precise information regarding properties of the layers, including anisotropy, is given in tabl. 1. On the front side of the considered reservoir vertical or horizontal producing well is located. Vertical well in all test cases completes both layers, horizontal well is variably placed in the upper or lower layer. The location of horizontal well is reflected in tabl. 1. The width of the reservoir is equal to 1200 m, the length of horizontal well - 600 m. On the back side of the reservoir vertical or horizontal injection well is symmetrically placed. P. 433
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