Analytical Solutions to Compute Optimal Well Configuration for Improved Gas Productivity
Authors: Rahim Zillur , Al-Kanaan Adnan , Al-Anazi Hamoud , Al-Omair Ahmed 
Published: March 2015

Drilling optimal number of wells is a critical task in the development of hydrocarbon reservoirs. Most important data needed to accomplish such an endeavor are the well deliverability potential that depend upon two major factors; (1) reservoir characteristics such as porosity, permeability, stresses, connectivity, reservoir pressure, condensate content, field extent, heterogeneity, etc., and (2) drilling, completion, and stimulation properties that include vertical, horizontal, and multi-lateral well configurations, open-hole, pre-perforated casing, or cemented liner completions, and single or multistage fracture (MSF) assemblies either open hole or plug and perf (P&F). Developing a gas field optimally is a tedious process that requires the involvement of multi-disciplinary highly skilled engineers and technologists, to ensure that production target and estimated ultimate recovery (EUR) are realistic and met.

In tight reservoirs, where conventional completion is inadequate to provide economic gas rates to produce the reserves in a timely manner, hydraulic fractures and often MSF techniques are induced to enhance well rate and long-term production sustainability. For a field where drilling horizontal wells and completing them with hydraulic fracturing are required, a comprehensive development program addressing various drilling and fracturing options must be initiated. These include drilling directions, lateral lengths, and drill in fluid characteristics, and pumping and fracturing fluid and proppant properties.

Much work has previously been done to address well productivity and optimal well spacing. This paper presents some quick and simple analytical solutions associating well geometry to productivity improvement. The impact of reservoir permeability, vertical anisotropy, net pay thickness, horizontal well length, fracture half-length, fracture conductivity, the number of finite transverse fractures induced in a horizontal wellbore, and many other relevant parameters on well productivity are depicted in this paper. Although the results obtained from the analytical solutions are optimistic as that they do not consider damage or operational issues and lost time, the data serve as excellent references to qualitatively assess the impact of different completion types on reservoir performance.

From mathematical study and field performance analyses, it can be concluded that horizontal wells perform better than hydraulically fractured vertical wells in relatively thin reservoirs. In thick reservoirs, particularly where the vertical permeability anisotropy is high, fracturing is needed to ensure vertical connectivity of the layers with the wellbore through the created fractures. In such conditions, hydraulically fractured vertical wells deliver at rates higher than the un-fractured, horizontal wells. Depending on reservoir conditions and well completion properties, a horizontal well with MSF may perform the best. In summary, there may be numerous development scenarios that must be considered through modeling and pilot testing to derive an effective and efficient field development program.