Genetic analysis and identification of low‐resistivity gas reservoirs of southwestern Sulige gas field in China

Abstract

Sulige gas field is the largest natural gas producing area in China. Due to the "water blocking" effect, the gas-water layer significantly influences the development of gas reservoirs. The existence of low-resistivity gas layers in the He 8 member of the Shihezi Formation in the southwestern Sulige gas field makes it challenging to distinguish the gas layers from the gas-water layers using conventional identification methods. To effectively identify the low-resistivity gas layers, their genetic mechanisms are analyzed by studying their lithology and pore structural characteristics based on the well logging and core experimental data. The low-resistivity gas layers are main affected by three causes: (1) electrical conductivity of argillaceous laminae in sandstone,(2) additional conductivity of clay minerals, and (3) high bound water saturation caused by the development of micropores and clay minerals in sandstone. Herein, to effectively distinguish the low-resistivity gas layer from the gas-water layer based on the genetic mechanism of the low-resistivity gas layer, first, a gas-bearing index was constructed to characterize the gas-bearing properties of the reservoir using the neutron logging and density logging curves after eliminating the influence of the dispersed shale. Second, a model to calculate the bound water saturation was constructed by selecting sensitive parameters with respect to the causes of bound water. Then, two plots, namely the gas-bearing index-natural gamma relative value cross-plot and the bound water content-porosity cross-plots, were constructed using the gas testing data, and the identification standard of the low-resistivity gas and gas-water layers was established. The interactive identification of the two cross-plots effectively distinguished the low-resistivity gas layer from the gaswater layer, thereby providing a basis for understanding the distribution of gas and water in the southwestern Sulige gas field, which may guide further exploration and the deployment of the development well pattern.

Matching of Water Breakthrough in Low Resistivity Oil Reservoir Using Permeability Anisotropy

“…Low resistivity in oil-bearing zones can lead to low contrast with water-bearing zones resulting in water saturation above 50% due to which they can be overlooked and bypassed [1,2]. Low resistivity and low contrasts can be associated with water salinity, clay type and micro-porosity, presence of conductive minerals, anisotropy, bed thickness and bed dip, drilling mud invasion, and logging tool resolution [1][2][3][4][5]. High salinity interstitial water causes low resistivity within the pay zone, while low salinity water can cause low contrast pays.…”

“…The main features of the low-resistivity pay are the abundant microporosity and high irreducible brine saturation resulting from cementation, clay coating, or clay bridging within pores and in finegrained sandstones [1,3]. Due to the lack of vertical resolution, the tools give an average resistivity measurement over the bedded sequences with laminar shales.…”

Matching of Water Breakthroughs in a Low-Resistivity Oil Reservoir Using Permeability Anisotropy

“…Shifting to the left indicates more water-wet and shifting to the right indicates more oil-wet contact between the rock and fluid. In the result, the following pairs of Kr with the specific crossing points were used to build the cases: Case 1 with Sw = 0.58 for (1, 2); Case 2 with Sw = 0.68 for (3,4); Case 3 with Sw = 0.48 for (5,6); Case 4 with Sw = 0.51 for (5, 2); Case 5 with Sw = 0.63 for (3, 2); Case 6 with Sw = 0.61 for (1,4); and Case 7 with Sw = 0.6 for (7,8). Crossing points and intervals of Sw are shown in Table 2 Thus, in Wells G, C and B, Kv/Kh = 1/1 provides a better match to the observed WPR because of the higher WPR than predicted using 1/10, while using 1/1 predicted a better match in Well G, because of lower WPR.…”

“…For example, no resistivity contrast between oil and water zones was observed for the reservoirs in Sudan which were bypassed in the past [2]. Low resistivity and low contrasts can be associated with water salinity, clay type, micro-porosity, the presence of conductive minerals, anisotropy, bed thickness, bed dip, drilling mud invasion, and logging tool resolution [1][2][3][4][5]. High-salinity interstitial water causes low resistivity within the pay zone, while low-salinity water can cause low contrast in the pay zone.…”

“…The main features of a low-resistivity pay zone are abundant microporosity and high irreducible brine saturation resulting from cementation, clay coating, or clay bridging within pores and in fine-grained sandstones [1,3]. Due to the lack of vertical resolution, the tools provide an average resistivity measurement over the bedded sequences with laminar shales.…”