Quantification of Overbalance-Induced Invasive Damage and the Estimation of Equivalent Skin Effect on Production

This paper describes the use of an under balanced drilling (UBD) system in a remote desert exploration well to directly evaluate the hydrocarbon reservoir inflow potential. The integrated sub-surface and well design team, planning, activity execution and results are discussed, with an assessment of the added value realized. The lessons learnt will be used to optimize the subsequent exploration and appraisal drilling activities, and in the longer term, optimization of subsequent field development activities.The vision is effectively to inflow test all available hydrocarbon reservoirs while drilling, thereby generating a pseudo reservoir inflow profile as a function of depth without the uncertainty of drilling induced formation damage. To achieve this objective, the available under balance drilling operating parameters and drilling fluids, were proactively controlled. This allowed the rig crew to manage the inflow while drilling, and near well bore skin damage, reducing the uncertainty to optimize the reservoir information acquired. The activity was executed in Q3 2007.Inflow testing in a known 'non damaged state' while drilling exploration and appraisal wells in desert areas remote from hydrocarbon processing and export infrastructure helps to reduce overall project risk. In low permeability reservoirs, over balance drilling induced near wellbore reservoir inflow impairment, has historically delivered inconclusive inflow data, resulting in higher project uncertainty and higher costs. UBD reservoir characterization information may be used to reduce subsequent well flow testing activities, which in turn, lowers project costs, speeds up the field appraisal process and reduces waste.

One Company's First Exploration UBD Well for Characterizing Low Permeability Reservoirs

Woodrow

Elliot

Pickles³

et al. 2008

Can UBD Flaring Reduce CO2 Emissions?

Elliott

Ramalho

2009

One of the perceived blockers for Underbalanced Drilling (UBD) operations globally is the necessity for flaring gas in most operations. As the world becomes more aware of the impact of CO2 emissions, oil and gas producers are implementing flaring reduction targets or, in some cases, flaring bans as a key component in their corporate strategies. Project teams make often argue that due to flaring requirements, UBD is not aligned with corporate directives and is is not environmentally acceptable. Thus the technology is eliminated as a considered option when finalizing field development plans. As the UBD industry has matured, use of UBD to increase production and reserve recovery from wells, which has resulted in a reduction of the number of wells needed to develop a field. Reservoir characterization from UBD has been used to better target wells in the reservoir, reducing the chances for "junk wells" with poor or no production. UBD is also used as an enabler to slim down wells with reduced steel requirements and to drill faster with reduced rig emissions. This paper will evaluate the trade off of increased CO2 emissions from UBD flaring against reduced CO2 emissions from fewer required wells, reduced steel requirements and reduced rig time. Case studies of several Shell UBD projects will be presented to determine the "CO2 cost neutral point" and if the projects reduced or increased global CO2 emissions. Conditions under which UBD may be justified to reduce project CO2 emissions will be determined and discussed.

Study of the Impact of Reduced Permeability Due to Near-Wellbore Damage on the Optimal Parameters of the Matrix Acidizing in Carbonate Rocks

Bekibayev

Beisembetov²,

Assilbekov³

et al. 2015

The pore space near the wellbore damaged with a time due to fine particles, chemical reactions, reservoir pressure difference, etc. As a result of the above mentioned factors reservoir properties of near-wellbore zone are altered, which negatively affects the productivity/injectivity of wells. One way to restore the reservoir properties of near-wellbore zone is matrix acidizing. An analysis of the literature review shows that there is a definite law of permeability reduction of the near-wellbore zone due to formation damage with the maximum reduction observed at the wellbore, and with the distance from the wellbore it restores to the original value. The influence of reservoir properties reduction due to damage on the effectiveness of the matrix acidizing is the subject of these studies, which based on numerical simulation of the dissolution of carbonate rock with hydrochloric acid. Numerical simulation of the dissolution of carbonate rock with hydrochloric acid is carried out using two-scale model in the polar coordinate system. To account the reduced reservoir properties due to formation damage of near-wellbore zone in numerical calculations special function fd is introduced. In this paper, a comparative analysis of results of dissolution modeling with the damage (fd ≠ 0) and without damage (fd = 0) function is shown. As a result of numerical simulations it was revealed that the dominant wormholes grows faster than non-dominant wormholes in the case with fd ≠ 0, which leads to a quick acid breakthrough. On the contrary, in the base case (fd = 0), non-dominant wormholes become relatively developed. Optimal Damköhler number for the base case is Daopt = 330, while fd ≠ 0 for the number is varying from 210 to 310 depending on the damage ratio Δεd and parameter λ. It was also shown that in the case of fd ≠ 0 wormhole grows faster than in the case of fd = 0. In the first case, the final skin factor is achieved when a volume of the injected acid is 75% less than amount required for the base case.