B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

1.9 Tutorial Topic 1 Questions 
Some questions taken or adapted from “Natural Gas Hydrates: A Guide for 
Engineers”, John Carroll. Note that answers obtained may vary depending on 
accuracy in reading the charts. 
1. Determine the hydrate formation pressure for ethane at 10 °C using the 
gas gravity method. [Answer: 1.25 MPa] 
2. Find the hydrate formation pressure for the following mixture at 14.2 °C 
using the gas gravity method. [Answer: 4 MPa] 
Methane
0.820 
Carbon Dioxide
0.126 
Hydrogen Sulphide 
0.054 
3. Repeat 2 using the K-factor method. [Answer: approx. 5 MPa] 
4. Find the amounts and compositions of all phases present when 
hydrate is formed from a mixture of 90.4 % methane and 9.6 % ethane 
at 2000 kPa and 275 K. [Answer: 86.5 % vapour, 13.5% solid; Vapour:
95% M, 5% E; Solid: 60% M, 40% E] 

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TOPIC 2: Oilfield Scale 
 
 
 
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NIVERSITY B41OA December 2018 v3 
2.1 Introduction 
This is an overview of the problems of oilfield mineral scale formation, both 
downhole and topside. Mineral scale may be deposited both downhole and 
topside throughout the water production lifetime of a producer well. 
In this topic, we describe the types of mineral scale that are formed, why these 
scales occur (the basic chemistry of scale formation) and how the field 
development scheme, in conjunction with the well types used, can affect both 
scale formation and scale prevention measures. 
Figure 1 shows examples of the type of production problem that can arise, if a 
potential scale problem is ignored: 
Figure 1: Scaled Pipes (https://fqechemicals.com/contaminants/barium-
sulfate-scale/) 
This clearly shows how scale can severely constrict a pipe. However, even 
more serious damage may occur should scale forms in Sub-Sea Safety Valves 
(SSSVs) and/or in topside chokes and valves. 
The most common mineral scales formed from produced waters, both in the 
North Sea and elsewhere are calcium carbonate (CaCO
3
) and barium sulphate 
(BaSO
4
) scales. 
(Vetter, 1982; Weintritt, 1967; Shen, 1983; Charleston, 1968; Cowan, 
1976; Hardy, 1994; Voloshin, 2003; Nasr –El-Din, 2003; Ramstad, 
2004) 
The origin of the carbonate scale is due to the presence of calcium and 
bicarbonate ions in the produced water which, when pressure falls, or other 
conditions change, may precipitate as calcite (CaCO
3
). 
Barium sulphate (BaSO
4
) is generally formed when there is co-production of 
formation water containing barium with seawater that is rich in sulphate ions. 

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TOPIC 2: Oilfield Scale 
 
 
 
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The following are the most common types of scale that have been 
encountered during both the production of North Sea formation water (FW) 
and formation water/seawater (FW/SW) mixtures.