Abstract
In complex and sensitive environments such as the northern Barents Sea, operations face multiple challenges, both technically and logistically. The use of logging while drilling (LWD) technology mitigates risks and assures acquisition of formation evaluation data in a complex trajectory. All data gathering was performed in LWD and provided the kernel for interpretation; alternate scenarios utilizing pipe conveyed wireline elevated risk factors as well as higher overall costs. Novel technology was required for this data acquisition, including fluid mapping while drilling (FMWD) that allows fluid identification with the use of downhole fluid analysis (DFA) using optical spectrometry as well as the retrieval of downhole fluid samples and a unique sourceless multifunction LWD tool delivering key data for the petrophysical evaluation.
This paper presents a case study of the first application of a combination of FMWD and a petrophysical LWD toolstring in the Barents Sea. An excellent contribution to the operator of the PL229 that have pushed the boundaries of the formation sampling while drilling and set the basis to challenge the potentiality of this technique and improve the knowledge of the methodology that are the ultimate goals of this paper.
Methods, procedures, process
Hydrocarbon exploration, production, and transport in the Barents Sea are challenging. The shallow and complex reservoirs are at low temperature and pressure, potentially with gas caps. The Goliat field is the first offshore oil development in this environment, producing from two reservoirs: Realgrunnen and Kobbe. As part of the Goliat field infill drilling campaign with the aim of adding reserves and increase production, PL229 license operator drilled a highly deviated pilot hole to confirm hydrocarbons contacts in the undrained Snadd formation, which lie between two producing reservoirs.
A successful data acquisition would not only provide information on the structure of the reservoir but would also assess the insitu movable fluid: type of hydrocarbon or water. FMWD allowed insitu fluid identification with the use of DFA, enabling RT evaluation of hydrocarbon composition as well as the filtrate contamination prior to filling the sampling bottles for further laboratory analysis. All data was acquired while drilling and using a comprehensive real-time visualization interface.
Results, observation, conclusion
Extensive prejob planning was conducted to optimize the operation. Dynamic fluid invasion simulations were used to estimate the required cleanup times to reach low contaminations. Simulations showed there was significant advantage in cleanup times when sampling soon after drilling.
Honoring the natural environment, a unique sourceless multifunction LWD tool was used to acquire data for petrophysical evaluation-GR, resistivity, radioisotope-free density and neutron porosity, elemental capture spectroscopy, and sigma.
Fluid mapping in a single run was key to efficiently resolve the insitu fluid type and composition. Critical hydrocarbon samples were collected soon after the formation was drilled to minimize mud filtrate invasion and reduce cleanup times. Multiple pressure measurements were acquired and six downhole fluid samples at low contamination (∼3% confirmed by laboratory) collected at several stations in variable mobilities. One scanning station was done at a zone were a physical sample was not required to confirm absence of gas cap. The DFA capabilities and ability to assess composition and control the fluid cleanup from surface allowed critical decisions to complete the acquisition program in this remote complex environment, all while drilling.
In conclusion, FMWD results facilitated the placement decisions of the horizontal drain in this reservoir.
This green BHA is unique in the LWD world. It eliminates radioactive source-handling and all related environmental risks to provide a comprehensive reservoir characterization. FMWD contributes formation pressure and fluid characterization and enables the physical capture of fluid samples in a single run.
The combination of these two technologies completed the formation and fluid evaluation needs in this remote and environmentally sensitive area while drilling.