A Comparative Study of Pseudo-Steady State Inflow Performance Between Horizontal and Vertical Gas Wells for Thin Reservoirs in the Niger Delta

Abstract

No horizontal gas well has been drilled in the Niger Delta since the inception of the technology and its first application in the region. The argument has been that the present vertical well technology, unlike oil, is adequate for gas production considering the relatively favourable petrophysical properties (e.g. permeability) and the usually significant pay thickness in the region. As a result, no evaluation of possible application of the horizontal well technology in gas production has been considered in the region. However, there are several pockets of gas resources trapped in highly heterogeneous reservoirs of less than 20ft within the region which pose deliverability challenge to the use of vertical wells in terms of drawdown and flow assurance. For a quick evaluation of the possible impact of horizontal technology on gas well deliverability in the Niger Delta, the effective wellbore radius concept was applied. This concept can be used for practical comparison in reservoirs and/or fields where no horizontal well has been drilled; particularly for small pay thicknesses. As a result, a comparative horizontal inflow model for pseudo-steady state radial flow was developed and validated using field data. It was observed that for thin anisotropic reservoirs, horizontal well performance is at least better than vertical well as much as the following conditions are satisfied: (1) h/L=0.1 for all the sensitized range of kh/kv, (2) h/L=0.2 for kh/kv ≤ 35, (3) h/L=0.3 for kh/kv ≤10, (4) h/L=0.4 for kh/kv ≤5 and (5) h/L=0.5 for kh/kv =2. Above the scaled aspect ratio boundaries, the deliverability ratio drops below one. Though the horizontal well deliverability is higher the impact of turbulence is seen to be in the range of 0.98 – 1.06 depending on the scaled aspect ratio; and the effect of reservoir anisotropy is between 2 – 80% for these thin reservoirs across typical ranges (200 ≥ kh/kv ≥ 2) seen in the Niger Delta. This paper presents the comparative model and its quick evaluation results with examples after due testing; considering the impact of non-Darcy turbulence, scaled aspect ratio and reservoir anisotropy.