New Laboratory Procedures For Evaluation of Drilling Induced Formation Damage And Horizontal Well Performance
Authors: Gruber N.G. , Adair K.L. 
Published: March 2010

Abstract
The popularity and breadth of application of horizontal well technology continues to expand. In some cases, however, horizontal well performance is disappointing - often as a result of drilling-induced formation damage. This damage may occur over the entire pay interval, or may occur in discreet intervals in varying degrees, depending on drilling conditions and changes in lithology along the wellbore. The resultant inflow may occur over one or more short segments rather than the entire horizontal wellbore. This limited drainage profile can lead to premature coning, and poorer than expected performance.
Formation damage is normally modelled using a skin factor, s, in the Darcy flow equation, assumed to be a constant. The skin value used, if not measured directly from pressure transient analysis. can be based on laboratory regain permeability tests conducted on core samples, These tests involve comparing the final (regain) permeability, after mud damage during leakoff, to the initial undamaged permeability. The final regain permeability is typically measured at some arbitrary flow rate (and therefore. some arbitrary drawdown), Our research indicates that this test method is over-simplified because it does not recognize the ariable nature in which wellbore cleanup occurs.
A more comprehensive test procedure has been developed for mud leakoff and regain permeability testing, whereby the regain permeability is measured at incrementally-increasing pressure differentials across the core. Imposing a pressure differential and measuring the resultant flow rate is consistent with applying a drawdown in the actual well. Application of the new test procedure with different drilling muds on several core plugs of varying permeability indicates that fluid inflow will not occur until a minimum "threshold pressure" is achieved" Lab results showed distinct differences in threshold pressure magnitude between different mud systems and rock permeability. Regain permeability improves with increasing pressure drop and with increasing volumetric throughput In some cases, it returns to its non-damaged value.
A finite-difference wellbore simulator has been developed, incorporating the dynamic cleanup effect, to model early-time transient productivity in a horizontal well. The model predicts significantly poorer flow-contribution profiles than might be expected from the flow-capacity (kh) profile. The model also demonstrates how uneven wellbore cleanup is more likely to result in shorter effective well lengths in low-viscosity, high-permeability (high flow-capacity) reservoirs. In these reservoirs, where horizontal wells are often drilled for coning mitigation, shorter effective well lengths can lead to unexpected premature coning.

Introduction
Drilling-induced formation damage is recognized as a potential cause of lost well productivity(1). With horizontal wells, formation damage presents increased difficulties because:formation damage is often more severe(2)stimulation costs to overcome damage are high.assessment of well performance and identification of skin damage is difficult.
Keelan and Koepf(1) emphasized the importance of performing core studies to evaluate formation damage prior to drilling. Several authors(l-8) used dynamic core-flow tests to characterize drilling fluid filtration behaviour, and to quantify damage caused by drilling mud following the filtration process. Marx and Rahman(4) emphasized the importance of performing core-flow tests at conditions that are close to borehole conditions.