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.

Understanding hydrocarbon migration and trapping is important since it can mean the difference between success and failure in exploration projects. The current understanding is that a capillary pressure change between a seal and a carrier bed (or reservoir) is the main factor responsible for the trapping. The current theory uses capillary pressure gradients under static (no flow) conditions to define the maximum amount of hydrocarbon that can be trapped under a particular seal. It assumes that at the very low flow rates encountered in secondary migration viscous pressure drops are negligible. Using numerical simulation and theoretical analysis, we show that, even at very low flow rates, viscous pressure drops are not negligible and that pressure gradients within phases can be substantially different from the static gradients. We present a theory that includes the effect of both the capillary and viscous forces. An innovative way of including the effect of capillary pressures in the method of characteristics is used to solve the migration and trapping problem. Migration and trapping are explained as a result of reflection and refraction of non-linear saturation waves from the heterogeneity boundaries. When viscous forces are included the seals can trap substantially more hydrocarbons than those predicted by the current theory. It is possible to classify seals into static and dynamic seals based on their capillary pressure curves and on the petrophysical properties of the carrier bed. In both cases, we are able to associate a time scale to the accumulation and indicate explanations for several other features commonly observed in secondary migration. The results from the proposed theory are confirmed using numerical simulations. Introduction Hydrocarbons (oil and gas) are formed by the decomposition of organic solids deposited in fine grained sediments, mostly shales. With subsequent burial, the pressure and temperature in these rocks increase and some of the bonds in the kerogen are broken to produce oil or gas. After their production, these hydrocarbons must be transported and concentrated into more porous and permeable regions to form hydrocarbon reservoirs. The movement of hydrocarbons just after their formation in the source rocks until they reach the more permeable rocks is called primary migration (Fig. 1). Primary migration finishes when hydrocarbons are expelled from the fine grained rocks into the large permeability rocks called carrier beds. The subsequent movement of hydrocarbons after they emerge from the source rock is called secondary migration (Fig. 1). There are different ways in which secondary migration can occur: in solution, as micelles or as a separate hydrocarbon phase. Most of the evidence points to separate phase migration. Even in the case when the migration occurs in other forms, the hydrocarbons must come out of solution to form traps. Thus, the final stage of secondary migration will always be a separate hydrocarbon phase migration. This paper assumes that all the migration occurs in a separate hydrocarbon phase, either an oil or a gas phase, depending on the temperature and pressure conditions and the composition of the fluids. As hydrocarbons enter the large pores of a carrier bed they may coalesce to form larger globules. These large globules will move up by buoyancy. Hydrocarbons move in these carrier beds until they reach locations where further movement is partially or totally stopped. The obstacles to the further movement of hydrocarbons are called seals. The region beneath the seal that contains the trapped hydrocarbons at high concentrations is called a hydrocarbon trap or a hydrocarbon reservoir (Fig. 1). No seal is perfect. They all fail under certain conditions, allowing the hydrocarbons to leak from the trap. Leakage is in effect a continuation of secondary migration although it is sometimes also referred as tertiary migration. After leaking from a trap, the hydrocarbons may trap under another seal or may ultimately seep to the surface. P. 395^

Volatile oil and gas condensate reservoirs were uncommon before 1930s. Since then they have been discovered with increasing frequency. This is attributed to the increasing drilling depths. Recently there has been a growing interest in these near critical fluids reservoirs. Large number of discoveries made it imperative to implement the relevant methods to deal with such reservoirs. Usual industrial practice in Pakistan is to use the P/Z plot to estimate the GIIP (Gas initially in place). This is the simplest method and does not require the set of PVT properties below the saturation pressure. However the P/Z method is limited in its application to only volumetric depletion type, dry gas, reservoirs. The general material balance equation gives reliable GIIP estimates, since it accounts for all drive mechanisms. Usually general material balance methods are not preferred in gas condensates, because these require standard PVT properties below the saturation pressure. These properties may be computed using Walsh-Towler algorithm, Whitson-Torp (K-value flash) method or Cubic Equation of State (EOS). This paper demonstrates the application of Walsh-Towler algorithm and K-value flash method (Whitson-Torp method) to calculate the standard PVT properties below the saturation pressure for a gas condensate reservoir. Then the application of Havlena-Odeh and Cole plot methods is exhibited to estimate the GIIP. The results are then compared with that of P/Z plot and an 11% over-prediction by P/Z plot was observed.

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