Organic Richness and Productivity Index relationship in Dual Porosity flow-system of gas and condensate Kerogen reservoirs of Najmah Formation, North Kuwait

Abstract

Abstract The Najmah formation of Jurassic age has been tested and found to be a prolific source-rock as well as a producer of gas, condensate and light oil in different wells discovered from various fields of North Kuwait. The challenges of this unconventional reservoir are its dual porosity nature and its relation to the flow controlling system in view of very low porosity and the dependency on natural fracture network system as flowing mechanism. The drill stem test (DST) results at some wells were quite successful without any stimulation, while at other wells the DSTs were unsuccessful in spite of advanced and repeated stimulations, thus categorizing the Najmah as a geologically-complex, naturally-fractured tight gas and condensate reservoir. Pressure transient analysis and flow regime interpretation of the successfully run Drill Stem Tested (DST) wells confirm the dual porosity flow-system and the fractured nature of the reservoir. In this paper, the authors will point out the relationship between organic richness (Total Organic Content, TOC) of lower Najmah sub-unit and Productivity Index (PI). The term "matrix-dominated" is used to represent units with some dual-pore systems showing a typical profile with initial fracture flow and stabilized matrix support. The term "fracture-dominated" is used for units flowing through a fracture network but with limited matrix support. The cross-plot shows that PI increases with higher values of TOC, up to 2.45% which is the upper limit of TOC window, within which Najmah has been successfully tested as dual pore-system, matrix-dominated Kerogen reservoir, while fracture-dominated dual pore-flow system lie as outliers. The authors strongly feel that this finding will tremendously change the current understanding of the prospectivity and predictive estimation of productivity of the Najmah formation in other fields in Kuwait as well as in other regions. Introduction The Jurassic formations in North Kuwait have proven hydrocarbon potentials and prospectivity for gas, condensate and light oil from composite Najmah-Sargelu and Marrat reservoirs in different fields of North Kuwait such as North-west Raudhatain, Raudhatain, Sabriyah, Umm Niqqa, Dhabi and Bahra (Figure 1). The Najmah formation has two informal members; generally known as lower, Najmah shale and upper, Najmah limestone. The lower member, represented by high total gamma ray values associated with high uranium on spectral gamma ray logs, is composed of highly organic-rich argillaceous and calcareous clay, is called the Najmah shale or Najmah kerogen. The term Najmah kerogen in this work is used interchangeably with Najmah shale. This Kerogenous unit is believed to be one of the main source rocks for the shallower and younger Cretaceous in most major oil reservoirs in Kuwait, as well as the deeper reservoirs. However the results of drill stem tests highlighted the problem of how to identify productive zones and sweeter areas in these unconventional reservoirs, challenged with their geologically complex and naturally fractured, tight nature. To date, 29 wells have been drilled in these six fields and 14 of them tested in composite Najmah-Sargelu reservoir, with a 50% success rate.