Many development projects will rely on producing through existing production facilities, which may not have been designed for sour hydrogen sulphide (H2S) service. This problem is compounded if production is routed to an NGL or GTL facility because even a tiny amount of H2S may dictate a prohibitively expensive upgrade.
Detecting the presence of H2S early in the life of a discovery can help to accurately assess the feasibility of a project, and determining if an offset discovery can be produced without a facility upgrade can economically make or break a project. Traditionally, operators have relied on well tests to determine H2S levels. In addition to the expenses associated with a well test, there is the ever-present issue of H2S scavenging. Many days of flow may be required in order to sufficiently passivate the metals so that an accurate H2S concentration can be determined.
Wireline formation testers have historically been regarded as a non-viable alternative. In this paper we are challenging this historical concept. By using carefully designed laboratory experiments; we have studied the H2S scavenging effects of different metals. The tests were conducted for different concentrations of H2S at a pre-defined flowrate and in the presence of water in order to quantify H2S scavenging effects. We have also identified all the components in the formation tester string, which could lead to scavenging. These parts were then redesigned and manufactured with the upgraded metals. This enhancement to wireline formation testers helps the technology to encompass a wider spectrum of functionalities.
In this paper, we will review the choice of materials, the verification test procedures and the laboratory test results. In addition we will discuss options for the design of the downhole tool in order to optimize sample recovery. We feel that this is a new level of refined utility for wireline formation testers in approaching well test functionalities.
H2S is an extremely hazardous, toxic compound that occurs in a number of natural and industrial environments. Naturally, it can be found in coal pits, sulfur springs, gas wells, and as a product of decaying sulfur-containing organic matter, particularly under low oxygen conditions. Industrial sources of H2S include petroleum and natural gas extraction and refining, pulp and paper manufacturing, rayon textile production, leather tanning, chemical manufacturing and waste disposal.
The Hydrogen Sulfide (H2S) content of subsurface hydrocarbon reservoirs has a profound impact on completion design and on project economics in general. The need for accurate determination of H2S concentration in the reservoir fluids to be produced is crucial. It may mean higher processing fees or a lower price for the oil or gas. It may also determine if it will be possible to access existing producing facilities or not and thus the feasibility of a project. The accurate measurement of H2S concentration in the reservoir fluid can often be critical to completion, surface production, and process design and is important for many reasons including the following:
Determine which (if any) HSE measures must be implemented for dealing with H2S at the various stages of exploration, appraisal, development, production, and abandonment of a given prospect.
Indicate the need for special metallurgical or process design to deal with certain levels of H2S in the presence of various other mitigating or accentuating factors.
Detect the onset and evolution of reservoir souring upon the implementation of water injection or other enhanced recovery techniques.
Determine the price of a unit hydrocarbon produced and its conditional reception at pre-existing pipelines or facilities.
Conventionally, the measurement of H2S concentration in fluid samples was limited to production tests. Wireline Formation Testers (WFT) were considered unusable for this purpose due to the relatively small pumped volumes as well as the partial-to-total loss of the gas by reaction with the metal components of the tools used in sampling. Analysis of the fluid samples will therefore generally underestimate the H2S content. Many practical challenges must be overcome to accurately determine H2S levels in formation tester samples.