Wilcox Formation Evaluation: Improved Procedures for Tights Gas-Sand Evaluation
Authors: Lewis D.J. , Perrin J.D. 
Published: December 2007

Summary
Risks in tight-gas-sand evaluation are reduced by defining relationshipsbetween pore geometry and critical water saturations. These results areintegrated with log interpretation to derive an estimated kh that comparesfavorably with a true kh from production tests. These procedures arepotentially applicable for evaluating other complex reservoirs.

Introduction
Tight gas sands. such as those common in the Lower Wilcox formation of Texas, are routinely difficult to identify accurately as candidates fortesting. completion, or abandonment. Several factors contribute to thisdifficulty.Porosity and permeability are controlled by complex and variablediagenesis that leads to a poor correlation between porosity and permeability.porosity and permeability.A dual-porosity system is present, wheredepositional and deagenetic clays, along with grain and cement dissolution, create isolated macro- and micropores that do not contribute to flowPore-lining clays are present and variable, and although they can preserveporosity and permeability by limiting quartz over-growths, they preserveporosity and permeability by limiting quartz over-growths, they also cancompletely fill pore throats. Clay types and their positions within the porestructure are not quantifiable from log responses.Critical water saturationis difficult to establish because the heterogeneity of rock types causesextreme variability in critical water saturations. This leads to someproductive rock types having water saturations approximately equal to otherrock types that are at residual gas saturation.Wilcox formation waterresistivity, Rw, frequently varies from zone to zone and is difficult todetermine petrophysically, especially in wells with oil-based mud. Obtainingpetrophysically, especially in wells with oil-based mud. Obtaining R fromporosity logs is difficult because a 100% wet zone is seldom available for thecalculation.Completions are difficult and variable because the pore systemis fragile and easily damaged by drilling and completion fluids.

Producing Behavior, Producing Behavior, Theory, and Definitions
Wilcox formation gas production is controlled primarily by the pore geometryand the amount of water present. This production pore geometry and the amountof water present. This production behavior assumes no retrograde condensatedropout in the reservoir and is characterized by two main factors: increasingwater saturation significantly decreases effective permeability and relativepermeability behavior changes as a function of pore geometry. permeabilitybehavior changes as a function of pore geometry. Relative permeability behaviorchanges with the amount of water present in the producing pores. Multiple typesof pore geometries can isolate water from main flow paths and reduce the amountof water available to flow. Understanding the differences betweenlaboratory-measured and air permeabilities and the actual effectivepermeability of a reservoir fluid is key to distinguishing nonproductive fromproductive intervals. In tight-gas-sand formations. permeability must bemeasured at in-situ conditions. including elevated confining pressure. porepressure, and water saturation. pressure. pore pressure, and water saturation. We define absolute permeability, k, as the gas permeability, k, of the dryrock (S = 0) at in-situ effective stress conditions (overburden minus porepressure). Effective permeability is the k at in-situ conditions. Relativepermeability is the ratio of effective to absolute permeability.

Methodology
A comprehensive approach that addressed complex formation characteristicswas required to improve evaluation of tight gas sands. To develop theprocedures, we established commercial criteria, measured core properties, measured pore geometry from core and cuttings, developed a k estimator frompore geometry and a k estimator from k, defined kg/k type curves, anddeveloped a type-curve estimator from pore geometry. First, we developedcommercial production criteria by examining well production characteristics. The criteria were calculated from pressure buildup data and four-pointproduction tests that provided the minimum kh required for both successfulnonstimulated and stimulated completions. Second, rock properties were measuredfrom core plugs. We measured unstressed and dry porosity and permeability, aswell as mercury-injection capillary porosity and permeability, as well asmercury-injection capillary pressure and drainage non-steady-state pulse-decaypermeability. We pressure and drainage non-steady-state pulse-decaypermeability. We took thin-sections from each plug and used image analysis tocollect pore geometry and compositional characteristics.
JPT
P. 724