Greater Unity a multilayered clastic reservoir in Sudan is a conglomeration of number of fault blocks- lacustrine deposits of late cretaceous age. Reservoir characteristics are mostly heterogeneous with varying degree of heterogeneities both vertically and horizontally. Reservoirs are highly undersaturated and have poor aquifer support. Rapid pressure decline was observed in early phase of production, severely affecting the performance of pumps resulting into frequent failures and causing sharp production decline. Water injection in low pressure mode was resorted in some blocks. Failure rates of ESP and PCP reduced significantly as dynamic fluid level (DFL) increased noticeably, provided sufficient submergence, and improvement in efficiency of the pumps.
Significant decline in injectivity in Aradeiba formation compelled to change the strategy of injection. Step rate tests were the guiding factor for selecting the low and high pressure injections and also stimulation. Paper discusses application of diagnostic methods like Hall plot, Jordan plot and other empirical relations using Pressure, injection and production data for understanding and improving the injection process. Profile modification for better conformance control gained early importance in view of smaller sizes of the pools.
Nonparametric statistical method known as Spearman rank analysis has been used to understand and analyze the degree of communication between injectors and producers. This analysis quickly identifies the communication between injectors and producers, or lack of communication and helps in understanding the response of injection. Preferential flow trends are reflected by the correlation in rates between injectors and producers along with lead time response of injection on production.
Paper illustrates the important ingredients which can add value to asset and improve the reserves and overall development strategy. Therefore, it is highlighted that success and failure of water injection project depends on why, when, where, what, how and how much to inject, plus what will happen to the formation once the water injection starts.
Introduction
Numbers of oil-bearing fields have been discovered in Muglad basin in the concession area operated by GNPOC. Areal extent of these fields varies from 0.5 Sq Km to 35–40 Sq Km. Some of the major fields are namely Unity, Toma South, Neem and Heglig. In place oil volume varies from 5 MMSTB to 500 MMSTB. Depth of oil-bearing formation varies from 1200m to 3100m. The Unity area is located in the upper Nile province of the Republic of Sudan, approximately 1,400km away from the coast of the Red Sea and 730km southwest to Khartoum. Greater Unity which comprises more than 20 fault- blocks is the biggest field in terms of oil inplace and number of wells (Fig-1). Greater Unity field was discovered in 1980 and was put on production in June 1999 and comprises of number of fault blocks. Main-Unity is the largest block representing ca. 40% of the total in-place. Other fault blocks are small to moderate in sizes. A number of satellite fields have been discovered nearby to these major fields. There are some isolated fields away from the existing fields. It covers a relatively wide area some 20 km2 comprising more than 20 discovered structures. The multi-layered oil-bearing formations were penetrated from as shallow as Ghazal at ∼1800 mkb to as deep as Bentiu at ∼ 2800 mkb. Four main reservoir groups have been encountered in these fields namely Ghazal, Zarqa, Aradeiba and Bentiu with several sub-units within the major reservoir group with varying fluid contacts. It is highly faulted with shifting fault planes with depth in some structures. The reservoir properties are generally moderate with moderate to light oil properties of 28–35 o API and relatively low oil viscosities of 2–15 cp. 2P STOIIP is around 1175 MMstb with EUR of 375 MMstb representing a RF of ∼ 32%. Greater Unity fields are developed by 100 producers supported by 23 injectors. All producers are supported by either Progressing Cavity Pump (PCP) or Electrical Submercible Pump (ESP). Fig-2 shows the historical performance of Greater Unity field.
