# Investigation on Interference Test for Wells Connected by a Large Fracture
## 1 Introduction  
The connectivity and permeable directivity of a reservoir are needed for the design of a development plan that accounts for such factors as a reasonable well pattern, the option of injection, and production wells for injection water or gas development [1; 2; 3; 4; 5]. The connectivity and permeable directivity are usually tested via interference test. The interference test is also widely used to investigate the properties of water and geothermal wells, except oil and gas wells [6; 7].  
Early in 1936, [PERSON] [8] proposed the type-curve of the interference test for homogeneous reservoirs but did not account for the impact of the wellbore storage effect and the skin effect, which were later studied by [PERSON] et al. [9]. In 1972, [PERSON] and [PERSON] [10] presented the curves of the interference test for vertically fractured wells. An interference test analysis model for complexreservoirs, such as composite reservoirs [11], two-layered commingled reservoirs [12], and a deformable reservoir [13] have also been proposed. [PERSON] [14] proposed a method to distinguish the spherical flow and linear flow for the interference test. Since the 1990s, the interference test analysis model for horizontal wells has become a hot topic [15; 16; 17], such as the models for anisotropic horizontal wells [18; 19], fractured horizontal wells [20], and multilayers horizontal wells [21; 22].  
The interference test can also be used to investigate the connectivity between an injection well and a production well, where interference test analysis models of a multiphase flow [23; 24; 25] are needed. [PERSON] et al. [26] and [PERSON] et al. [27] studied interference test analysis models that consider complex flow mechanisms for unconventional reservoirs.  
Most existing interference test analysis models consider homogeneous reservoirs with different boundaries, but most reservoirs are heterogeneous and have fractures. If the geological information is complete, a numerical interference test analysis can be performed for a heterogeneous reservoir. A fractured reservoir can be characterized by a multimedium model or discrete fracture model depending on the density of fractures. [PERSON] and [PERSON] analyzed the interference test of double porosity medium reservoirs using the Warren-Root model for reservoirs with intensive fractures [28; 29; 30]. The multimedium model does not work for reservoirs with sparse fractures. For example, the reservoirs at Halahatang Oilfield, Tarim Basin in China are occurrent along faults resembling a string of beads. Although [PERSON] and [PERSON] [31] investigated the effect of discrete fractures on well test curves, there are few studies on the impact of large fractures on the interference test.  
In the past decade, multi-stage hydraulic fracturing horizontal wells have been widely used in shale gas development. Frequently, hydraulic fracturing causes pressure communication in adjacent horizontal wells [32; 33; 34]. Interference test wells are also used to test their connectivity [35]. The existing analysis models consider the flow of the entire reservoir and are generally solved through numerical or semi-numerical methods [36; 37]. However, it is not necessary to consider the matrix flow in a fractured reservoir when the test time is short.  
Therefore, a one-dimensional equation of flow in porous media is presented in this paper to study the pressure transient of wells connected by large fractures. Using its solution, an extremum equation and interference test type-curve for wells connected by a large fracture (WsCLF) were obtained and compared with the type-curve of homogeneous reservoirs and double porosity reservoirs. The results were verified against interference test data.