Proper Fluid Pretreatment to Minimize Hydrogen Sulphide Dangers
Authors: Samuels A. , Wendt R.P. 
Published: March 2010

Introduction

The potential hazards in the drilling, completion and/or servicing of wells containing hydrogen sulphide are described. A new generalized mud theory is used to calculate the level of corrosive or poisonous gas contamination due" to varying amounts of hydrogen sulphide and carbon dioxide entering a mud stream as dependent on the mud"s buffering capacity and alkalinity. It is demonstrated that an intrusion 0/ gas contain ing as little as 0.5 mole per cent hydrogen sulphide can create hazardous conditions. Even highly aklaline muds are predicted to have severely limited ability to neutralize acid gas intrusions and cannot be relied on for safety.

The important need for pretreatment of the muds to remove hydrogen sulphide rather than merely neutralize it is described. Published laboratory kick simulation experiments and documented field results show that proper scavenger pretreatment and monitoring of the mud can protect against a well kick producing 10 million cubic feet (2.83 × 105m3) per day of sweet gas and 500,000 cubic feet (1.42 × 100m") per day of hydrogen sulphide.

Introduction

Over the past several years, considerable attention has been devoted to the subject of hydrogen sulphide dangers during drilling, completion and warkover operations. These dangers are well understood and have been the subject of numerous articles(1–7). What constitutes various degrees of potential hazards and their proper pretreatment is not well understood. Potential hazard levels have been minimally defined by the Texas Railroad Commission in Rule No. 36 regarding production operations(8). However, no satisfactory definitions of conditions leading to dangerous risks in drilling have been established. Treatment recommendations for these various potential hazards have never been determined and the advantages or limitations of various treatment methods are not clear This has led to unsafe practices which can greatly increase the risks in operations involving hydrogen sulphide.

For example, alkalinity control may be perfectly safe to neutralize the H2S drilled up in the pore space of a rock containing small amounts of hydrogen sulphide. However, the same treatment may prove dangerous for a gas formation containing high H2S concentrations. Add the potential of gas-cut mud and increasing concentrations of carbon dioxide and the danger increases substantially.

One of the authors (Richard P. Wendt) has previously presented a theory of the potential hazards of alka1inity control(9,10,11). Further refinements of this theory were discussed in the presentation to the Second Latin American Drilling Congress, held in September 1980 in Mexico City, and are included in this paper. It can be inferred from this work that the need for non-alkalinity methods of controlling hydrogen sulphide is greater than previously suspected. The treatment levels needed to reacted remove hydrogen sulphide (rather than simply neutralize it) are dependent on the efficiency and effectiveness of the various materials and the ability to easily add them in sufficient amounts without adversely affecting mud rheology. Methods of measuring H2S effects are normally only minimally applied in the field.