Background: Sufficient literature has been published about Pre-Darcy flow in non-petroleum disciplines. Investigators dissent about the significance of deviation of Darcy's law at very low fluid velocities. Most of their investigations are based on coarse, unconsolidated porous media with an aqueous fluid. However little has been published regarding the same for consolidated oil and gas reservoirs. If a significant departure from Darcy's law is observed, then this could have multiple implications on: reservoir limit tests, under prediction of reserves, unrecognized prospecting opportunities etc. Methods: This study performs a comprehensive review of the literature. Experiments were conducted to confirm the presence and the significance of Pre-Darcy flow effect in petroleum rocks. Results: The review of literature and experiments indicate the presence of Pre-Darcy effect. Contributing factors to Pre-Darcy effect are discussed and some reasons causing this effect are postulated. The experiments also show that this effect is significant. Conclusion: Pre-Darcy effect is significant because it is the dominant flow regime in typical petroleum reservoirs.

Hydraulic fracturing is being widely employed to augment wells’ productivity, thus facilitating proper depletion of the reservoir fluids and adding to the recoverable hydrocarbon reserves. The benefits of hydraulic fracturing are particularly pronounced in reservoirs exhibiting low permeability, high skin and, in case of gas condensate reservoirs, near wellbore condensate banking. A gas condensate field, code name ‘Delta’, has been producing from sandstone reservoirs for the last two decades. Most of the wells have been suffering from low productivity primarily due to a relatively low reservoir permeability (0.5 to 10mD) and high drilling induced skin. The problem has become more pronounced with the depletion of reservoir pressure resulting in condensate drop out around the wellbore. This paper details the envisioned economic incentives and the post-frac deliverability results from the recent hydraulic fracturing campaign carried out in a gas condensate field. The paper highlights the operational challenges encountered and the evolution of the hydraulic fracturing treatment design, execution and post-frac completion / flow-back strategy based on our experiences that contributed towards a successful and challenging campaign.

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We investigate the impact of nonlinearity of high and low velocity flows on the well productivity index (PI). Experimental data shows the departure from the linear Darcy relation for high and low velocities. High-velocity (post-Darcy) flow occurring near wells and fractures is described by Forchheimer equations and is relatively well-studied. While low velocity flow receives much less attention, there is multiple evidence suggesting the existence of pre-Darcy effects for slow flows far away from the well. This flow is modeled via pre-Darcy equation. We combine all three flow regimes, pre-Darcy, Darcy and post-Darcy, under one mathematical formulation dependent on the critical transitional velocities. This allows to use our previously developed framework to obtain the analytical formulas for the PI for the cylindrical reservoir. We study the impact of pre-Darcy effect on the PI of steady-state flow depending on the well-flux and the parameters of the equations.

Carbonate reservoirs in Northern Pakistan are characterized by tight limestone.In these reservoirs, fractures are important for production and reservoirmodeling. This paper addresses problems related to subsurface fracture analysisbased mainly on image logs. Natural fractures occur as systematic and unsystematic sets of definite andrandom orientation respectively. The subsurface analysis of fractures useselectrical and acoustic image logs to characterize fractures as either naturalor induced features. They are classified as conductive or resistive features, representing possibly open or closed (mineralized) fractures, respectively.Using image logs, natural fractures are interpreted and classifieddescriptively to be continuous or discontinuous features representingsystematic fractures or classified as chicken-wire (microfractures) fracturesrepresenting unsystematic sets. Statistical analysis of fractures is used toclassify them into geometrical and genetic sets as longitudinal (extensional), transverse (tensional), and oblique (shear) to the structure. Transversefractures are known generally as most open. They develop parallel to themaximum horizontal in-situ stress and extend deep into the structure.Longitudinal fractures, those parallel to the fold axes, are observed toproduce hydrocarbons in several fields in Northern Pakistan. Fracture densityimpacts production and reserves calculations. However, fracture density isstrongly influenced by the lithology and layer thickness. Widely spacedfractures are observed in massive carbonate reservoirs, and closely spacedfractures of narrower aperture are observed in laminated strata. Thus, individual fractures in massive carbonates require to be identified for theirimpact on production. Fractures are observed to occur as discontinuous featuresof right- or left-stepping geometry and as en echelon features of significantlywider aperture in shear bands. These features together with vugs and leachedfeatures may provide zones of higher porosity, permeability, and storagecapacity with isolated distribution in tight carbonates. Therefore, knowledgeabout fracture occurrence and distribution is important to predict sweet spotsfor drilling and field development.